Investments - Credit Ratings (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) security	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Fixed Maturities Table [Line Items]
Total other-than-temporary impairment losses	$ 1,506	$ 0
Number of securities with other-than-temporary impairment losses | security	2
Fixed Maturities, Amortized cost	$ 1,530,689		$ 1,813,426
Fixed Maturities, Fair value	$ 1,508,544		$ 1,835,518
Fixed maturities, % of Total fair value	100.00%		100.00%
AAA
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 97,181		$ 99,212
Fixed Maturities, Fair value	$ 92,229		$ 99,542
Fixed maturities, % of Total fair value	6.10%		5.40%
AA Plus, AA, AA Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 87,667		$ 101,491
Fixed Maturities, Fair value	$ 82,028		$ 101,467
Fixed maturities, % of Total fair value	5.40%		5.50%
A Plus, A, A Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 357,080		$ 540,002
Fixed Maturities, Fair value	$ 343,162		$ 549,479
Fixed maturities, % of Total fair value	22.80%		29.90%
BBB Plus, BBB, BBB Minus
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 382,271		$ 438,731
Fixed Maturities, Fair value	$ 371,173		$ 445,202
Fixed maturities, % of Total fair value	24.60%		24.30%
BB Plus or Lower
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 25,784		$ 5,773
Fixed Maturities, Fair value	$ 18,684		$ 5,481
Fixed maturities, % of Total fair value	1.20%		0.30%
U.S. treasury bonds
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 84,959		$ 94,921
Fixed Maturities, Fair value	$ 86,143		$ 95,625
Fixed maturities, % of Total fair value	5.70%		5.20%
U.S. agency bonds
Schedule of Fixed Maturities Table [Line Items]
Fixed Maturities, Amortized cost	$ 495,747		$ 533,296
Fixed Maturities, Fair value	$ 515,125		$ 538,722
Fixed maturities, % of Total fair value	34.20%		29.40%