Fair Value Measurement (Assets and Liabilities Measured at Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Equity securities	$ 410,906	$ 387,274
Other investments	80,466	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.	201,154	204,719
Level 3 [Member] \| Corporate debt with limited observable inputs [Member]
Assets:
Total assets categorized within the fair value hierarchy	17,849	14,810
Level 3 [Member] \| Other asset-backed securities [Member]
Assets:
Total assets categorized within the fair value hierarchy	3,005	3,007
Fair Value, Measurements, Recurring [Member]
Assets:
Other investments	30,814	31,501
Total assets categorized within the fair value hierarchy	3,251,528	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.	201,154	204,719
Total assets at fair value, including NAV value not within fair value hierarchy	3,452,682	3,678,984
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury obligations [Member]
Assets:
Fixed maturities, available for sale	143,453	146,539
Fair Value, Measurements, Recurring [Member] \| U.S. Government-sponsored enterprise obligations [Member]
Assets:
Fixed maturities, available for sale	20,192	30,235
Fair Value, Measurements, Recurring [Member] \| State and municipal bonds [Member]
Assets:
Fixed maturities, available for sale	721,836	800,463
Fair Value, Measurements, Recurring [Member] \| Corporate debt, multiple observable inputs [Member]
Assets:
Fixed maturities, available for sale	1,294,879	1,264,181
Fair Value, Measurements, Recurring [Member] \| Corporate debt with limited observable inputs [Member]
Assets:
Fixed maturities, available for sale	17,849	14,810
Fair Value, Measurements, Recurring [Member] \| Residential mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	206,415	217,906
Fair Value, Measurements, Recurring [Member] \| Agency commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	11,960	12,783
Fair Value, Measurements, Recurring [Member] \| Other commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	17,157	19,611
Fair Value, Measurements, Recurring [Member] \| Other asset-backed securities [Member]
Assets:
Fixed maturities, available for sale	112,366	106,878
Fair Value, Measurements, Recurring [Member] \| Financial [Member]
Assets:
Equity securities	82,324	81,749
Fair Value, Measurements, Recurring [Member] \| Utilities/Energy [Member]
Assets:
Equity securities	44,687	52,869
Fair Value, Measurements, Recurring [Member] \| Consumer oriented [Member]
Assets:
Equity securities	56,079	61,284
Fair Value, Measurements, Recurring [Member] \| Industrial [Member]
Assets:
Equity securities	49,635	54,265
Fair Value, Measurements, Recurring [Member] \| Bond funds [Member]
Assets:
Equity securities	116,025	90,002
Fair Value, Measurements, Recurring [Member] \| All other [Member]
Assets:
Equity securities	62,156	47,105
Fair Value, Measurements, Recurring [Member] \| Short-term investments [Member]
Assets:
Short-term investments	263,701	442,084
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets:
Other investments	963	1,956
Total assets categorized within the fair value hierarchy	665,761	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	2,406	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	82,324	81,749
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Utilities/Energy [Member]
Assets:
Equity securities	44,687	52,869
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Consumer oriented [Member]
Assets:
Equity securities	56,079	61,284
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Industrial [Member]
Assets:
Equity securities	49,635	54,265
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Bond funds [Member]
Assets:
Equity securities	116,025	79,843
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| All other [Member]
Assets:
Equity securities	62,156	27,181
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Short-term investments [Member]
Assets:
Short-term investments	251,486	437,580
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets:
Other investments	29,846	29,542
Total assets categorized within the fair value hierarchy	2,564,908	2,657,379
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| U.S. Treasury obligations [Member]
Assets:
Fixed maturities, available for sale	143,453	146,539
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| U.S. Government-sponsored enterprise obligations [Member]
Assets:
Fixed maturities, available for sale	20,192	30,235
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| State and municipal bonds [Member]
Assets:
Fixed maturities, available for sale	721,836	800,463
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Corporate debt, multiple observable inputs [Member]
Assets:
Fixed maturities, available for sale	1,292,473	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	206,415	217,906
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Agency commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	11,960	12,783
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Other commercial mortgage-backed securities [Member]
Assets:
Fixed maturities, available for sale	17,157	19,611
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Other asset-backed securities [Member]
Assets:
Fixed maturities, available for sale	109,361	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	12,215	4,504
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets:
Other investments	5	3
Total assets categorized within the fair value hierarchy	20,859	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	17,849	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	3,005	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

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- 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

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- Definition

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

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- Definition

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

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- 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.

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Amount of investments classified as other.

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

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