Investments (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,295,086	$ 1,623,319
Gross Unrealized Gains	561	823
Gross Unrealized Losses	(9,224)	(7,298)
Fair Value	1,286,423	1,616,844
Agency securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	356,504	441,315
Gross Unrealized Gains	361	509
Gross Unrealized Losses	(3,268)	(2,760)
Fair Value	353,597	439,064
Corporate securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	625,874	810,444
Gross Unrealized Gains	193	268
Gross Unrealized Losses	(4,277)	(3,020)
Fair Value	621,790	807,692
Municipal securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,500	3,965
Gross Unrealized Gains	5	2
Gross Unrealized Losses	0	(2)
Fair Value	2,505	3,965
Government securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	310,208	367,595
Gross Unrealized Gains	2	44
Gross Unrealized Losses	(1,679)	(1,516)
Fair Value	$ 308,531	$ 366,123

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).

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-Section 50
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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).

+ References

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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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