Investments - Credit Ratings (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Fixed Maturities Table [Line Items]
Total other-than-temporary impairment losses	$ 479	$ 0	$ 479	$ 0
Securities considered other-than-temporarily impaired | security	1		1
Fixed Maturities, Amortized cost	$ 3,900,136		$ 3,900,136		$ 3,797,098
Fixed Maturities, Fair value	$ 3,801,294		$ 3,801,294		$ 3,833,142
Fixed maturities, % of Total fair value	100.00%		100.00%		100.00%
AAA
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 166,512		$ 166,512		$ 156,631
Fixed Maturities, Fair value	$ 163,099		$ 163,099		$ 159,682
Fixed maturities, % of Total fair value	4.30%		4.30%		4.20%
AA Plus, AA, AA Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 174,111		$ 174,111		$ 146,264
Fixed Maturities, Fair value	$ 169,207		$ 169,207		$ 147,054
Fixed maturities, % of Total fair value	4.40%		4.40%		3.80%
A Plus, A, A Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 1,136,634		$ 1,136,634		$ 1,089,230
Fixed Maturities, Fair value	$ 1,113,761		$ 1,113,761		$ 1,106,430
Fixed maturities, % of Total fair value	29.30%		29.30%		28.90%
BBB Plus, BBB, BBB Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 851,348		$ 851,348		$ 824,351
Fixed Maturities, Fair value	$ 839,940		$ 839,940		$ 845,244
Fixed maturities, % of Total fair value	22.10%		22.10%		22.10%
BB Plus or Lower
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 62,102		$ 62,102		$ 49,979
Fixed Maturities, Fair value	$ 64,277		$ 64,277		$ 51,776
Fixed maturities, % of Total fair value	1.70%		1.70%		1.30%
U.S. treasury bonds
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 125		$ 125		$ 35,093
Fixed Maturities, Fair value	$ 124		$ 124		$ 35,097
Fixed maturities, % of Total fair value	0.00%		0.00%		0.90%
U.S. agency bonds
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 1,509,304		$ 1,509,304		$ 1,495,550
Fixed Maturities, Fair value	$ 1,450,886		$ 1,450,886		$ 1,487,859
Fixed maturities, % of Total fair value	38.20%		38.20%		38.80%