Investments - Credit Ratings (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule of Fixed Maturities Table [Line Items]
Total other-than-temporary impairment losses	$ 0	$ 0	$ 0	$ 0
Fixed Maturities, Amortized cost	2,942,207,000		2,942,207,000		$ 4,125,661,000
Fixed Maturities, Fair value	$ 2,971,598,000		$ 2,971,598,000		$ 4,049,580,000
Fixed maturities, % of Total fair value	100.00%		100.00%		100.00%
AAA
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 117,628,000		$ 117,628,000		$ 137,172,000
Fixed Maturities, Fair value	$ 117,891,000		$ 117,891,000		$ 135,119,000
Fixed maturities, % of Total fair value	4.00%		4.00%		3.30%
AA Plus, AA, AA Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 154,929,000		$ 154,929,000		$ 183,142,000
Fixed Maturities, Fair value	$ 156,037,000		$ 156,037,000		$ 178,674,000
Fixed maturities, % of Total fair value	5.20%		5.20%		4.40%
A Plus, A, A Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 784,152,000		$ 784,152,000		$ 1,132,993,000
Fixed Maturities, Fair value	$ 800,009,000		$ 800,009,000		$ 1,113,710,000
Fixed maturities, % of Total fair value	26.90%		26.90%		27.50%
BBB Plus, BBB, BBB Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 491,469,000		$ 491,469,000		$ 866,043,000
Fixed Maturities, Fair value	$ 496,589,000		$ 496,589,000		$ 848,348,000
Fixed maturities, % of Total fair value	16.70%		16.70%		21.00%
BB Plus or Lower
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 25,900,000		$ 25,900,000		$ 52,229,000
Fixed Maturities, Fair value	$ 25,694,000		$ 25,694,000		$ 52,290,000
Fixed maturities, % of Total fair value	0.90%		0.90%		1.30%
U.S. treasury bonds
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 365,953,000		$ 365,953,000		$ 138,625,000
Fixed Maturities, Fair value	$ 367,103,000		$ 367,103,000		$ 139,072,000
Fixed maturities, % of Total fair value	12.40%		12.40%		3.40%
U.S. agency bonds
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 1,002,176,000		$ 1,002,176,000		$ 1,615,457,000
Fixed Maturities, Fair value	$ 1,008,275,000		$ 1,008,275,000		$ 1,582,367,000
Fixed maturities, % of Total fair value	33.90%		33.90%		39.10%