Investments - Credit Ratings (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Schedule of Fixed Maturities Table [Line Items]
OTTI	$ 0	$ 0	$ 0	$ 0
Fixed Maturities, Amortized cost	4,825,866,000		4,825,866,000		$ 4,757,854,000
Fixed Maturities, Fair value	$ 4,865,455,000		$ 4,865,455,000		$ 4,737,801,000
Fixed maturities, % of Total fair value	100.00%		100.00%		100.00%
AAA [Member]
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 167,065,000		$ 167,065,000		$ 170,515,000
Fixed Maturities, Fair value	$ 169,687,000		$ 169,687,000		$ 171,090,000
Fixed maturities, % of Total fair value	3.50%		3.50%		3.60%
AA Plus, AA, AA Minus [Member]
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 207,671,000		$ 207,671,000		$ 238,315,000
Fixed Maturities, Fair value	$ 211,290,000		$ 211,290,000		$ 237,169,000
Fixed maturities, % of Total fair value	4.30%		4.30%		5.00%
A Plus, A, A Minus [Member]
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 1,351,748,000		$ 1,351,748,000		$ 1,386,023,000
Fixed Maturities, Fair value	$ 1,368,370,000		$ 1,368,370,000		$ 1,374,860,000
Fixed maturities, % of Total fair value	28.20%		28.20%		29.00%
BBB Plus, BBB, BBB Minus [Member]
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 1,151,997,000		$ 1,151,997,000		$ 1,053,529,000
Fixed Maturities, Fair value	$ 1,169,178,000		$ 1,169,178,000		$ 1,047,376,000
Fixed maturities, % of Total fair value	24.00%		24.00%		22.20%
BB Plus or Lower [Member]
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 126,505,000		$ 126,505,000		$ 165,768,000
Fixed Maturities, Fair value	$ 127,857,000		$ 127,857,000		$ 167,753,000
Fixed maturities, % of Total fair value	2.60%		2.60%		3.50%
U.S. treasury bonds [Member]
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 5,191,000		$ 5,191,000		$ 5,186,000
Fixed Maturities, Fair value	$ 5,361,000		$ 5,361,000		$ 5,413,000
Fixed maturities, % of Total fair value	0.10%		0.10%		0.10%
U.S. agency bonds [Member]
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 1,815,689,000		$ 1,815,689,000		$ 1,738,518,000
Fixed Maturities, Fair value	$ 1,813,712,000		$ 1,813,712,000		$ 1,734,140,000
Fixed maturities, % of Total fair value	37.30%		37.30%		36.60%