Investments (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 260,676	$ 1,164,848
Gross Unrealized Gains	370	544
Gross Unrealized Losses	(1,105)	(7,458)
Fair Value	259,941	1,157,934
Agency securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	69,987	314,982
Gross Unrealized Gains	249	333
Gross Unrealized Losses	(289)	(2,367)
Fair Value	69,947	312,948
Corporate securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	118,689	612,698
Gross Unrealized Gains	99	116
Gross Unrealized Losses	(490)	(4,156)
Fair Value	118,298	608,658
Municipal securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	0	2,500
Gross Unrealized Gains	0	4
Gross Unrealized Losses	0	0
Fair Value	0	2,504
Government securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	72,000	234,668
Gross Unrealized Gains	22	91
Gross Unrealized Losses	(326)	(935)
Fair Value	$ 71,696	$ 233,824

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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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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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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