Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	$ 5,706,656	$ 5,270,798
Unrealized Gains	231,328	47,369
Unrealized Losses	(7,747)	(45,067)
Debt Securities, Available-for-sale	5,930,237	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	116,555	120,092
Unrealized Gains	4,357	1,810
Unrealized Losses	(12)	(592)
Debt Securities, Available-for-sale	120,900	121,310
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	20,947	23,202
Unrealized Gains	573	36
Unrealized Losses	0	(107)
Debt Securities, Available-for-sale	21,520	23,131
AFS Fixed Income Securities [Member] \| Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,127,709	1,121,615
Unrealized Gains	62,910	19,485
Unrealized Losses	(4)	(2,631)
Debt Securities, Available-for-sale	1,190,615	1,138,469
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,762,981	1,639,852
Unrealized Gains	78,345	5,521
Unrealized Losses	(2,399)	(27,965)
Debt Securities, Available-for-sale	1,838,927	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	764,038	720,193
Unrealized Gains	8,998	4,112
Unrealized Losses	(4,653)	(6,943)
Debt Securities, Available-for-sale	768,383	717,362
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	524,441	527,409
Unrealized Gains	31,435	3,417
Unrealized Losses	(145)	(3,748)
Debt Securities, Available-for-sale	555,731	527,078
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,389,985	1,118,435
Unrealized Gains	44,710	12,988
Unrealized Losses	(534)	(3,081)
Debt Securities, Available-for-sale	$ 1,434,161	$ 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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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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