Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	$ 9,852,710	$ 9,950,773
Available-for-sale investments, amortized cost	10,607,974	11,030,800
Available-for-sale investments, gross unrealized holding gains	361,990	224,073
Available-for-sale investments, gross unrealized holding losses	(53,257)	(133,989)
Available-for-sale fixed maturity investments, estimated fair value	10,162,419	10,043,188
Available-for-sale investments, estimated fair value	10,916,707	11,120,884
U.S. Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	272,837	248,286
Available-for-sale fixed maturity investments, gross unrealized holding gains	1,048	308
Available-for-sale fixed maturity investments, gross unrealized holding losses	(1,113)	(1,952)
Available-for-sale fixed maturity investments, estimated fair value	272,772	246,642
U.S. Government-Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	356,042	357,765
Available-for-sale fixed maturity investments, gross unrealized holding gains	12,121	5,671
Available-for-sale fixed maturity investments, gross unrealized holding losses	(843)	(4,114)
Available-for-sale fixed maturity investments, estimated fair value	367,320	359,322
Obligations Of States, Municipalities And Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	4,206,957	4,285,068
Available-for-sale fixed maturity investments, gross unrealized holding gains	156,087	96,730
Available-for-sale fixed maturity investments, gross unrealized holding losses	(6,191)	(28,868)
Available-for-sale fixed maturity investments, estimated fair value	4,356,853	4,352,930
Foreign Governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	1,481,680	1,482,826
Available-for-sale fixed maturity investments, gross unrealized holding gains	133,912	98,356
Available-for-sale fixed maturity investments, gross unrealized holding losses	(8,693)	(21,578)
Available-for-sale fixed maturity investments, estimated fair value	1,606,899	1,559,604
Commercial Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	1,690,466	1,691,572
Available-for-sale fixed maturity investments, gross unrealized holding gains	17,161	3,154
Available-for-sale fixed maturity investments, gross unrealized holding losses	(24,016)	(44,527)
Available-for-sale fixed maturity investments, estimated fair value	1,683,611	1,650,199
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	879,245	886,501
Available-for-sale fixed maturity investments, gross unrealized holding gains	15,176	6,170
Available-for-sale fixed maturity investments, gross unrealized holding losses	(4,544)	(12,499)
Available-for-sale fixed maturity investments, estimated fair value	889,877	880,172
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	13,185	19,614
Available-for-sale fixed maturity investments, gross unrealized holding gains	9	7
Available-for-sale fixed maturity investments, gross unrealized holding losses	(105)	(213)
Available-for-sale fixed maturity investments, estimated fair value	13,089	19,408
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	952,298	979,141
Available-for-sale fixed maturity investments, gross unrealized holding gains	26,346	13,234
Available-for-sale fixed maturity investments, gross unrealized holding losses	(6,646)	(17,464)
Available-for-sale fixed maturity investments, estimated fair value	971,998	974,911
Total Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity investments, amortized cost	9,852,710	9,950,773
Available-for-sale fixed maturity investments, gross unrealized holding gains	361,860	223,630
Available-for-sale fixed maturity investments, gross unrealized holding losses	(52,151)	(131,215)
Available-for-sale fixed maturity investments, estimated fair value	10,162,419	10,043,188
Short-Term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale investments, amortized cost	755,264	1,080,027
Available-for-sale investments, gross unrealized holding gains	130	443
Available-for-sale investments, gross unrealized holding losses	(1,106)	(2,774)
Available-for-sale investments, estimated fair value	$ 754,288	$ 1,077,696

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (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)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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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).

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-Publisher FASB
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