Fair Value Measurement (Assets and Liabilities Measured at Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Equity securities	$ 391,206	$ 387,274
Other investments	80,576	81,892
LP/LLC interests carried at NAV which approximates fair value. These interests, reported as a part of Investment in unconsolidated subsidiaries, are not categorized within the fair value hierarchy.	199,696	204,719
Level 3 [Member] \| Corporate debt with limited observable inputs [Member]
Assets:
Total assets categorized within the fair value hierarchy	18,914	14,810
Level 3 [Member] \| Other asset-backed securities [Member]
Assets:
Total assets categorized within the fair value hierarchy	3,002	3,007
Fair Value, Measurements, Recurring [Member]
Assets:
Other investments	29,978	31,501
Total assets categorized within the fair value hierarchy	3,305,954	3,474,265
LP/LLC interests carried at NAV which approximates fair value. These interests, reported as a part of Investment in unconsolidated subsidiaries, are not categorized within the fair value hierarchy.	199,696	204,719
Total assets at fair value, including NAV value not within fair value hierarchy	3,505,650	3,678,984
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury obligations [Member]
Assets:
Fixed maturities, available for sale	148,211	146,539
Fair Value, Measurements, Recurring [Member] \| U.S. Government-sponsored enterprise obligations [Member]
Assets:
Fixed maturities, available for sale	28,701	30,235
Fair Value, Measurements, Recurring [Member] \| State and municipal bonds [Member]
Assets:
Fixed maturities, available for sale	770,061	800,463
Fair Value, Measurements, Recurring [Member] \| Corporate debt, multiple observable inputs [Member]
Assets:
Fixed maturities, available for sale	1,287,059	1,264,181
Fair Value, Measurements, Recurring [Member] \| Corporate debt with limited observable inputs [Member]
Assets:
Fixed maturities, available for sale	18,914	14,810
Fair Value, Measurements, Recurring [Member] \| Residential mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	207,906	217,906
Fair Value, Measurements, Recurring [Member] \| Agency commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	12,688	12,783
Fair Value, Measurements, Recurring [Member] \| Other commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	19,697	19,611
Fair Value, Measurements, Recurring [Member] \| Other asset-backed securities [Member]
Assets:
Fixed maturities, available for sale	111,192	106,878
Fair Value, Measurements, Recurring [Member] \| Financial [Member]
Assets:
Equity securities	84,052	81,749
Fair Value, Measurements, Recurring [Member] \| Utilities/Energy [Member]
Assets:
Equity securities	51,304	52,869
Fair Value, Measurements, Recurring [Member] \| Consumer oriented [Member]
Assets:
Equity securities	60,686	61,284
Fair Value, Measurements, Recurring [Member] \| Industrial [Member]
Assets:
Equity securities	52,497	54,265
Fair Value, Measurements, Recurring [Member] \| Bond funds [Member]
Assets:
Equity securities	92,753	90,002
Fair Value, Measurements, Recurring [Member] \| All other [Member]
Assets:
Equity securities	49,914	47,105
Fair Value, Measurements, Recurring [Member] \| Short-term investments [Member]
Assets:
Short-term investments	280,341	442,084
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets:
Other investments	1,270	1,956
Total assets categorized within the fair value hierarchy	672,365	799,066
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| U.S. Treasury obligations [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| U.S. Government-sponsored enterprise obligations [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| State and municipal bonds [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Corporate debt, multiple observable inputs [Member]
Assets:
Fixed maturities, available for sale	2,395	2,339
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Corporate debt with limited observable inputs [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Residential mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Agency commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Other commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Other asset-backed securities [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Financial [Member]
Assets:
Equity securities	84,052	81,749
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Utilities/Energy [Member]
Assets:
Equity securities	51,304	52,869
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Consumer oriented [Member]
Assets:
Equity securities	60,686	61,284
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Industrial [Member]
Assets:
Equity securities	52,497	54,265
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Bond funds [Member]
Assets:
Equity securities	92,753	79,843
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| All other [Member]
Assets:
Equity securities	49,914	27,181
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Short-term investments [Member]
Assets:
Short-term investments	277,494	437,580
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets:
Other investments	27,805	29,542
Total assets categorized within the fair value hierarchy	2,610,770	2,657,379
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| U.S. Treasury obligations [Member]
Assets:
Fixed maturities, available for sale	148,211	146,539
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| U.S. Government-sponsored enterprise obligations [Member]
Assets:
Fixed maturities, available for sale	28,701	30,235
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| State and municipal bonds [Member]
Assets:
Fixed maturities, available for sale	770,061	800,463
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Corporate debt, multiple observable inputs [Member]
Assets:
Fixed maturities, available for sale	1,284,664	1,261,842
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Corporate debt with limited observable inputs [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Residential mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	207,906	217,906
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Agency commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	12,688	12,783
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Other commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	19,697	19,611
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Other asset-backed securities [Member]
Assets:
Fixed maturities, available for sale	108,190	103,871
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Financial [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Utilities/Energy [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Consumer oriented [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Industrial [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Bond funds [Member]
Assets:
Equity securities	0	10,159
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| All other [Member]
Assets:
Equity securities	0	19,924
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Short-term investments [Member]
Assets:
Short-term investments	2,847	4,504
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets:
Other investments	903	3
Total assets categorized within the fair value hierarchy	22,819	17,820
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| U.S. Treasury obligations [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| U.S. Government-sponsored enterprise obligations [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| State and municipal bonds [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Corporate debt, multiple observable inputs [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Corporate debt with limited observable inputs [Member]
Assets:
Fixed maturities, available for sale	18,914	14,810
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Residential mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Agency commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Other commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Other asset-backed securities [Member]
Assets:
Fixed maturities, available for sale	3,002	3,007
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Financial [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Utilities/Energy [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Consumer oriented [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Industrial [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Bond funds [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| All other [Member]
Assets:
Equity securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Short-term investments [Member]
Assets:
Short-term investments	$ 0	$ 0

Assets, Fair Value Disclosure, Including NAV Value Not Within Fair Value Hierarchy

No definition available.

Name:	pra_AssetsFairValueDisclosureIncludingNAVValueNotWithinFairValueHierarchy
Namespace Prefix:	pra_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investments in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

Name:	us-gaap_AlternativeInvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investments classified as other.

+ References

+ Details

Name:	us-gaap_OtherInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investments in trading equity securities and other forms of trading securities that provide ownership interests.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a),1(b))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

+ Details

Name:	us-gaap_TradingSecuritiesEquity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant