Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	$ 5,216,964	$ 5,220,527
Unrealized Gains	25,153	130,058
Unrealized Losses	(51,961)	(5,358)
AFS securities, fair value	5,190,156	5,345,227
AFS Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost		5,076,716
Unrealized Gains		90,867
Unrealized Losses		(5,061)
AFS securities, fair value		5,162,522
AFS Fixed Income Securities [Member] \| U.S. Government and Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	97,304	49,326
Unrealized Gains	306	647
Unrealized Losses	(1,148)	(233)
AFS securities, fair value	96,462	49,740
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	18,015	18,040
Unrealized Gains	88	526
Unrealized Losses	(120)	(11)
AFS securities, fair value	17,983	18,555
AFS Fixed Income Securities [Member] \| Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,161,322	1,539,307
Unrealized Gains	10,848	44,245
Unrealized Losses	(7,261)	(582)
AFS securities, fair value	1,164,909	1,582,970
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,662,613	1,588,339
Unrealized Gains	6,342	30,891
Unrealized Losses	(19,302)	(1,762)
AFS securities, fair value	1,649,653	1,617,468
AFS Fixed Income Securities [Member] \| Collateralized Loan Obligations and Other Asset Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	772,738	789,152
Unrealized Gains	4,571	6,508
Unrealized Losses	(2,540)	(202)
AFS securities, fair value	774,769	795,458
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	495,208	382,727
Unrealized Gains	328	1,563
Unrealized Losses	(5,987)	(841)
AFS securities, fair value	489,549	383,449
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,009,764	709,825
Unrealized Gains	2,670	6,487
Unrealized Losses	(15,603)	(1,430)
AFS securities, fair value	$ 996,831	714,882
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost		143,811
Unrealized Gains		39,191
Unrealized Losses		(297)
AFS securities, fair value		182,705
Equity Securities [Member] \| Common Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost		129,696
Unrealized Gains		38,287
Unrealized Losses		(226)
AFS securities, fair value		167,757
Equity Securities [Member] \| Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost		14,115
Unrealized Gains		904
Unrealized Losses		(71)
AFS securities, fair value		$ 14,948

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

No definition available.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

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+ References

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+ References

No definition available.

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