Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 367,161
Due after one year through five years	1,900,096
Due after five years through ten years	930,951
Due after ten years	1,132,469
Total debt securities with single maturity date, amortized cost	4,330,677
Amortized Cost	5,357,436	$ 5,196,784
Fair Value
Due in one year or less	367,474
Due after one year through five years	1,930,664
Due after five years through ten years	976,031
Due after ten years	1,203,911
Total debt securities with single maturity date, fair value	4,478,080
Total at end of period	5,504,823	5,151,987
Asset backed securities (“ABS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	215,016
Amortized Cost	215,016	111,953
Fair Value
Total debt securities without single maturity date, fair value	217,701
Total at end of period	217,701	112,033
Residential mortgage backed securities (“RMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	213,024
Amortized Cost	213,024	189,238
Fair Value
Total debt securities without single maturity date, fair value	208,497
Total at end of period	208,497	178,961
Commercial mortgage backed securities (“CMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	268,189
Amortized Cost	268,189	276,352
Fair Value
Total debt securities without single maturity date, fair value	271,889
Total at end of period	271,889	267,660
Collateralized loan obligations (“CLO”)
Amortized cost
Total debt securities without single maturity date, amortized cost	330,530
Amortized Cost	330,530	310,587
Fair Value
Total debt securities without single maturity date, fair value	328,656
Total at end of period	$ 328,656	$ 305,295