Investments - Schedule of Investments Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		$ 2,942,207	$ 3,109,980
Fixed Maturities, available-for-sale, Gross unrealized gains			10,995
Fixed Maturities, available-for-sale, Gross unrealized losses			(69,407)
AFS fixed maturities		2,971,598	3,051,568
Fixed Maturities, held-to-maturity, Original or amortized cost		0	1,015,681
Fixed Maturities, held-to-maturity, Gross unrealized gains			3,872
Fixed Maturities, held-to-maturity, Gross unrealized losses			(21,541)
Fixed maturities, held-to-maturity, fair value			998,012
Investments, Original or Amortized Cost Basis		2,942,207	4,125,661
Investments, Gross Unrealized Gains		48,707	14,867
Investments, Gross Unrealized Losses		(19,316)	(90,948)
Investments, Fair Value		2,971,598	4,049,580
Carrying value of held-to-maturity securities at the time of transfer	$ 1,011,878
Held-to-maturity securities transferred to available-for-sale securities, unrealized gain	$ 14,230
U.S. treasury bonds
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		365,953	138,625
Fixed Maturities, available-for-sale, Gross unrealized gains		1,153	448
Fixed Maturities, available-for-sale, Gross unrealized losses		(3)	(1)
AFS fixed maturities		367,103	139,072
U.S. agency bonds – mortgage-backed
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		950,301	1,485,716
Fixed Maturities, available-for-sale, Gross unrealized gains		9,299	3,491
Fixed Maturities, available-for-sale, Gross unrealized losses		(3,203)	(36,073)
AFS fixed maturities		956,397	1,453,134
U.S. agency bonds – other
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		51,875	129,741
Fixed Maturities, available-for-sale, Gross unrealized gains		4	40
Fixed Maturities, available-for-sale, Gross unrealized losses		(1)	(548)
AFS fixed maturities		51,878	129,233
Non-U.S. government and supranational bonds
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		29,321	11,212
Fixed Maturities, available-for-sale, Gross unrealized gains		313	66
Fixed Maturities, available-for-sale, Gross unrealized losses		(298)	(1,206)
AFS fixed maturities		29,336	10,072
Asset-backed securities
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		220,938	216,072
Fixed Maturities, available-for-sale, Gross unrealized gains		681	425
Fixed Maturities, available-for-sale, Gross unrealized losses		(553)	(1,415)
AFS fixed maturities		221,066	215,082
Corporate bonds
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		1,310,870	1,128,614
Fixed Maturities, available-for-sale, Gross unrealized gains		37,115	6,525
Fixed Maturities, available-for-sale, Gross unrealized losses		(15,258)	(30,164)
AFS fixed maturities		1,332,727	1,104,975
Fixed Maturities, held-to-maturity, Original or amortized cost			957,845
Fixed Maturities, held-to-maturity, Gross unrealized gains			3,872
Fixed Maturities, held-to-maturity, Gross unrealized losses			(20,990)
Fixed maturities, held-to-maturity, fair value			940,727
Municipal bonds
Schedule of Investments [Line Items]
Total AFS fixed maturities, Original or amortized cost		12,949
Fixed Maturities, available-for-sale, Gross unrealized gains		142
Fixed Maturities, available-for-sale, Gross unrealized losses		0
AFS fixed maturities		$ 13,091
Fixed Maturities, held-to-maturity, Original or amortized cost			57,836
Fixed Maturities, held-to-maturity, Gross unrealized gains			0
Fixed Maturities, held-to-maturity, Gross unrealized losses			(551)
Fixed maturities, held-to-maturity, fair value			$ 57,285