Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	$ 5,571,491	$ 5,270,798
Unrealized Gains	195,287	47,369
Unrealized Losses	(8,880)	(45,067)
Debt Securities, Available-for-sale	5,757,898	5,273,100
AFS Fixed Income Securities [Member] \| U.S. Government and Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	143,461	120,092
Unrealized Gains	3,683	1,810
Unrealized Losses	(20)	(592)
Debt Securities, Available-for-sale	147,124	121,310
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	20,995	23,202
Unrealized Gains	494	36
Unrealized Losses	0	(107)
Debt Securities, Available-for-sale	21,489	23,131
AFS Fixed Income Securities [Member] \| Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,056,507	1,121,615
Unrealized Gains	54,516	19,485
Unrealized Losses	(6)	(2,631)
Debt Securities, Available-for-sale	1,111,017	1,138,469
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,733,319	1,639,852
Unrealized Gains	64,620	5,521
Unrealized Losses	(4,402)	(27,965)
Debt Securities, Available-for-sale	1,793,537	1,617,408
AFS Fixed Income Securities [Member] \| Collateralized Loan Obligations and Other Asset Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	762,896	720,193
Unrealized Gains	8,649	4,112
Unrealized Losses	(3,941)	(6,943)
Debt Securities, Available-for-sale	767,604	717,362
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	541,093	527,409
Unrealized Gains	24,737	3,417
Unrealized Losses	(237)	(3,748)
Debt Securities, Available-for-sale	565,593	527,078
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,313,220	1,118,435
Unrealized Gains	38,588	12,988
Unrealized Losses	(274)	(3,081)
Debt Securities, Available-for-sale	$ 1,351,534	$ 1,128,342

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

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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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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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