Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	$ 5,463,273	$ 5,270,798
Unrealized Gains	118,373	47,369
Unrealized Losses	(14,288)	(45,067)
AFS securities, fair value	5,567,358	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	125,664	120,092
Unrealized Gains	1,766	1,810
Unrealized Losses	(341)	(592)
AFS securities, fair value	127,089	121,310
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	20,238	23,202
Unrealized Gains	299	36
Unrealized Losses	(11)	(107)
AFS securities, fair value	20,526	23,131
AFS Fixed Income Securities [Member] \| Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,110,504	1,121,615
Unrealized Gains	39,936	19,485
Unrealized Losses	(197)	(2,631)
AFS securities, fair value	1,150,243	1,138,469
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,651,773	1,639,852
Unrealized Gains	32,355	5,521
Unrealized Losses	(6,113)	(27,965)
AFS securities, fair value	1,678,015	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	722,573	720,193
Unrealized Gains	5,050	4,112
Unrealized Losses	(5,383)	(6,943)
AFS securities, fair value	722,240	717,362
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	557,148	527,409
Unrealized Gains	11,867	3,417
Unrealized Losses	(815)	(3,748)
AFS securities, fair value	568,200	527,078
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,275,373	1,118,435
Unrealized Gains	27,100	12,988
Unrealized Losses	(1,428)	(3,081)
AFS securities, fair value	$ 1,301,045	$ 1,128,342

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

+ Details

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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.

+ Details

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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