Investments - Fair Value and Unrealized Losses (Details) $ in Thousands	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Fixed maturities, Fair value
Less than 12 Months, Fair value	$ 2,828,442	$ 1,230,177
12 Months or More, Fair value	805,523	907,155
Total, Fair value	3,633,965	2,137,332
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(55,776)	(12,193)
12 months or more, Unrealized losses	(35,769)	(28,251)
Total, Unrealized losses	$ (91,545)	$ (40,444)
Number of securities in an unrealized loss position | security	466	244
Investments fair value	$ 3,633,965	$ 2,137,332
Investments, unrealized losses	$ 91,545	$ 40,444
Number of securities in an unrealized loss position for 12 months or greater | security	93	100
Investments fair value, unrealized loss position for 12 months or greater	$ 805,523	$ 907,155
Investments in unrealized loss position for 12 months or greater, unrealized losses	35,769	28,251
U.S. treasury bonds [Member]
Fixed maturities, Fair value
Less than 12 Months, Fair value	4,233	0
12 Months or More, Fair value	582	587
Total, Fair value	4,815	587
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(9)	0
12 months or more, Unrealized losses	(19)	(13)
Total, Unrealized losses	(28)	(13)
U.S. agency bonds – mortgage-backed [Member]
Fixed maturities, Fair value
Less than 12 Months, Fair value	1,358,831	842,000
12 Months or More, Fair value	499,399	539,704
Total, Fair value	1,858,230	1,381,704
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(30,870)	(6,920)
12 months or more, Unrealized losses	(21,400)	(12,845)
Total, Unrealized losses	(52,270)	(19,765)
U.S. agency bonds – other [Member]
Fixed maturities, Fair value
Less than 12 Months, Fair value	34,773	19,816
12 Months or More, Fair value	0	0
Total, Fair value	34,773	19,816
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(170)	(163)
12 months or more, Unrealized losses	0	0
Total, Unrealized losses	(170)	(163)
Non-U.S. government and supranational bonds [Member]
Fixed maturities, Fair value
Less than 12 Months, Fair value	3,136	12,825
12 Months or More, Fair value	24,084	15,253
Total, Fair value	27,220	28,078
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(64)	(971)
12 months or more, Unrealized losses	(1,531)	(765)
Total, Unrealized losses	(1,595)	(1,736)
Asset-backed securities [Member]
Fixed maturities, Fair value
Less than 12 Months, Fair value	131,222	68,703
12 Months or More, Fair value	5,005	3,017
Total, Fair value	136,227	71,720
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(1,142)	(150)
12 months or more, Unrealized losses	(65)	(49)
Total, Unrealized losses	(1,207)	(199)
Corporate bonds [Member]
Fixed maturities, Fair value
Less than 12 Months, Fair value	1,236,397	247,341
12 Months or More, Fair value	276,453	348,594
Total, Fair value	1,512,850	595,935
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(23,138)	(3,905)
12 months or more, Unrealized losses	(12,754)	(14,579)
Total, Unrealized losses	(35,892)	(18,484)
Municipal bonds [Member]
Fixed maturities, Fair value
Less than 12 Months, Fair value	59,850	39,492
12 Months or More, Fair value	0	0
Total, Fair value	59,850	39,492
Fixed maturities, Unrealized losses
Less than 12 months, Unrealized losses	(383)	(84)
12 months or more, Unrealized losses	0	0
Total, Unrealized losses	$ (383)	$ (84)