Investments (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) security	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,375,382,000		$ 5,562,550,000
Fair Value	5,458,846,000		5,737,892,000
Credit allowance on available for sale fixed income securities	0
Other-than-temporary impairment losses recorded in other comprehensive income			0
Assets held by insurance regulators	14,200,000		$ 13,900,000
Impairment charges recognized in net realized gains (losses) on securities with the intent to sell	$ (300,000)
Net impairment losses recognized in earnings		$ (100,000)
Collateral, market value to principal balance of FHLB (as a percent)	102.00%
FHLB advance, collateral pledged	$ 164,900,000
Number of securities in unrealized loss position (in securities) | security	321		217
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,375,382,000		$ 5,562,550,000
Gross Unrealized Gains	157,550,000		185,244,000
Gross Unrealized Losses	(74,086,000)		(9,902,000)
Fair Value	5,458,846,000		5,737,892,000
Proceeds from sales of fixed income securities	212,800,000	106,000,000.0
Gross realized gains, fixed income securities	5,100,000	700,000
Gross realized losses, fixed income securities	(1,300,000)	$ (1,300,000)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	197,330,000		195,176,000
Gross Unrealized Gains	2,408,000		1,237,000
Gross Unrealized Losses	(21,000)		(210,000)
Fair Value	199,717,000		196,203,000
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,614,562,000		1,555,394,000
Gross Unrealized Gains	99,532,000		99,328,000
Gross Unrealized Losses	(3,258,000)		(857,000)
Fair Value	1,710,836,000		1,653,865,000
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,508,051,000		2,711,910,000
Gross Unrealized Gains	44,386,000		76,220,000
Gross Unrealized Losses	(41,335,000)		(3,008,000)
Fair Value	2,511,102,000		2,785,122,000
Asset backed securities (“ABS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	210,930,000		227,376,000
Gross Unrealized Gains	2,031,000		2,466,000
Gross Unrealized Losses	(6,311,000)		(178,000)
Fair Value	206,650,000		229,664,000
Residential mortgage backed securities (“RMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	259,641,000		271,384,000
Gross Unrealized Gains	6,032,000		429,000
Gross Unrealized Losses	(758,000)		(3,227,000)
Fair Value	264,915,000		268,586,000
Commercial mortgage backed securities (“CMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	268,598,000		274,234,000
Gross Unrealized Gains	3,161,000		5,531,000
Gross Unrealized Losses	(3,172,000)		(779,000)
Fair Value	268,587,000		278,986,000
Collateralized loan obligations (“CLOs”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	316,270,000		327,076,000
Gross Unrealized Gains	0		33,000
Gross Unrealized Losses	(19,231,000)		(1,643,000)
Fair Value	$ 297,039,000		$ 325,466,000