Investments (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 76,904	$ 1,164,848
Gross Unrealized Gains	8	544
Gross Unrealized Losses	(166)	(7,458)
Fair Value	76,746	1,157,934
Agency securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,681	314,982
Gross Unrealized Gains	0	333
Gross Unrealized Losses	(2)	(2,367)
Fair Value	1,679	312,948
Corporate securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	61,044	612,698
Gross Unrealized Gains	1	116
Gross Unrealized Losses	(160)	(4,156)
Fair Value	60,885	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	14,179	234,668
Gross Unrealized Gains	7	91
Gross Unrealized Losses	(4)	(935)
Fair Value	$ 14,182	$ 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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