Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 375,772
Due after one year through five years	1,748,834
Due after five years through ten years	948,568
Due after ten years	1,148,093
Total debt securities with single maturity date, amortized cost	4,221,267
Amortized Cost	5,229,535	$ 5,196,784
Fair Value
Due in one year or less	375,241
Due after one year through five years	1,753,881
Due after five years through ten years	966,862
Due after ten years	1,195,465
Total debt securities with single maturity date, fair value	4,291,449
Total at end of period	5,287,360	5,151,987
Asset backed securities (“ABS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	217,469
Amortized Cost	217,469	111,953
Fair Value
Total debt securities without single maturity date, fair value	218,481
Total at end of period	218,481	112,033
Residential mortgage backed securities (“RMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	188,201
Amortized Cost	188,201	189,238
Fair Value
Total debt securities without single maturity date, fair value	180,485
Total at end of period	180,485	178,961
Commercial mortgage backed securities (“CMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	272,074
Amortized Cost	272,074	276,352
Fair Value
Total debt securities without single maturity date, fair value	269,283
Total at end of period	269,283	267,660
Collateralized loan obligations (“CLO”)
Amortized cost
Total debt securities without single maturity date, amortized cost	330,524
Amortized Cost	330,524	310,587
Fair Value
Total debt securities without single maturity date, fair value	327,662
Total at end of period	$ 327,662	$ 305,295