Investments - Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
HTM fixed maturities, Amortized cost
Due in one year or less	$ 20,755
Due after one year through five years	285,460
Due after five years through ten years	439,252
Due after ten years	5,087
Total before securities without single maturities, Amortized cost	750,554
Fixed Maturities, held-to-maturity, Amortized cost	750,554
HTM fixed maturities, Fair value
Due in one year or less	20,714
Due after one year through five years	292,104
Due after five years through ten years	449,764
Due after ten years	5,075
Total before securities without single maturities, Fair value	767,657
Fixed maturities, held-to-maturity securities, fair value	767,657	$ 766,135
AFS fixed maturities, Amortized cost
Due in one year or less	57,366
Due after one year through five years	647,554
Due after five years through ten years	1,332,950
Due after five years through ten years	32,152
Total before securities without single maturities, Amortized cost	2,070,022
Fixed Maturities, available-for-sale, Original or amortized cost	3,993,941	4,005,642
AFS fixed maturities, Fair value
Due in one year or less	49,562
Due after one year through five years	634,020
Due after five years through ten years	1,332,615
Due after ten years	32,072
Total before securities without single maturities, Fair value	2,048,269
Fixed Maturities, available-for-sale, Fair value	3,967,907	3,971,666
U.S. agency bonds – mortgage-backed [Member]
HTM fixed maturities, Amortized cost
Amortized cost of securities without single maturities	0
HTM fixed maturities, Fair value
Fair value of securities without single maturities	0
AFS fixed maturities, Amortized cost
Amortized cost of securities without single maturities	1,679,527
Fixed Maturities, available-for-sale, Original or amortized cost	1,679,527	1,720,436
AFS fixed maturities, Fair value
Fair value of securities without single maturities	1,671,475
Fixed Maturities, available-for-sale, Fair value	1,671,475	1,716,038
Asset-backed securities [Member]
HTM fixed maturities, Amortized cost
Amortized cost of securities without single maturities	0
HTM fixed maturities, Fair value
Fair value of securities without single maturities	0
AFS fixed maturities, Amortized cost
Amortized cost of securities without single maturities	244,392
Fixed Maturities, available-for-sale, Original or amortized cost	244,392	217,232
AFS fixed maturities, Fair value
Fair value of securities without single maturities	248,163
Fixed Maturities, available-for-sale, Fair value	$ 248,163	$ 220,876