Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 372,478
Due after one year through five years	1,928,972
Due after five years through ten years	1,001,486
Due after ten years	1,100,219
Total debt securities with single maturity date, amortized cost	4,403,155
Amortized Cost	5,473,893	$ 5,196,784
Fair Value
Due in one year or less	373,388
Due after one year through five years	1,966,387
Due after five years through ten years	1,059,774
Due after ten years	1,184,735
Total debt securities with single maturity date, fair value	4,584,284
Total at end of period	5,660,992	5,151,987
Asset backed securities (“ABS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	212,792
Amortized Cost	212,792	111,953
Fair Value
Total debt securities without single maturity date, fair value	215,546
Total at end of period	215,546	112,033
Residential mortgage backed securities (“RMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	248,098
Amortized Cost	248,098	189,238
Fair Value
Total debt securities without single maturity date, fair value	246,457
Total at end of period	246,457	178,961
Commercial mortgage backed securities (“CMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	280,364
Amortized Cost	280,364	276,352
Fair Value
Total debt securities without single maturity date, fair value	286,812
Total at end of period	286,812	267,660
Collateralized loan obligations (“CLOs”)
Amortized cost
Total debt securities without single maturity date, amortized cost	329,484
Amortized Cost	329,484	310,587
Fair Value
Total debt securities without single maturity date, fair value	327,893
Total at end of period	$ 327,893	$ 305,295