Investments (Cumulative Noncredit Portion of Other Than Temporary Impairment Recognized in Other Comprehensive Income and Change in Fair Value Since Other Than Temporary Impairment) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Amortized cost	$ 655,692	$ 315,164
OTTI recognized in other comprehensive income	(171,760)	(173,974)
Change in fair value since OTTI was recognized	211,282	210,484
Fair value	695,214	351,674
Corporate Securities
Investment Holdings [Line Items]
Amortized cost	389,729	13,015
OTTI recognized in other comprehensive income	(3,700)	(4,263)
Change in fair value since OTTI was recognized	10,041	10,739
Fair value	396,070	19,491
Residential Mortgage Backed Securities
Investment Holdings [Line Items]
Amortized cost	255,611	297,582
OTTI recognized in other comprehensive income	(168,060)	(168,355)
Change in fair value since OTTI was recognized	202,923	201,620
Fair value	290,474	330,847
Commercial Mortgage Backed Securities
Investment Holdings [Line Items]
Amortized cost	7,823
OTTI recognized in other comprehensive income	0
Change in fair value since OTTI was recognized	0
Fair value	7,823
Other Asset Backed Securities
Investment Holdings [Line Items]
Amortized cost	2,529	4,567
OTTI recognized in other comprehensive income	0	(1,356)
Change in fair value since OTTI was recognized	(1,682)	(1,875)
Fair value	$ 847	$ 1,336