Investments (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) security	Jun. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,037,701,000		$ 6,037,701,000		$ 5,562,550,000
Allowance for Expected Credit Loss	(362,000)		(362,000)
Fair Value	6,344,718,000		6,344,718,000		5,737,892,000
Other-than-temporary impairment losses recorded in other comprehensive income					0
Assets held by insurance regulators	14,100,000		14,100,000		$ 13,900,000
Increase in credit allowance for the period	400,000		$ 400,000
Collateral, market value to principal balance of FHLB (as a percent)			102.00%
FHLB advance, collateral pledged	$ 165,700,000		$ 165,700,000
Number of securities in unrealized loss position (in securities) | security	197		197		217
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,037,701,000		$ 6,037,701,000		$ 5,562,550,000
Allowance for Expected Credit Loss	(362,000)		(362,000)
Gross Unrealized Gains	316,146,000		316,146,000		185,244,000
Gross Unrealized Losses	(8,767,000)		(8,767,000)		(9,902,000)
Fair Value	6,344,718,000		6,344,718,000		5,737,892,000
Proceeds from sales of fixed income securities			713,100,000	$ 201,400,000
Gross realized gains, fixed income securities	3,400,000	$ 3,300,000	15,900,000	5,300,000
Gross realized losses, fixed income securities	800,000	$ 700,000	5,900,000	3,000,000.0
Impairment charges recognized on securities with the intent to sell	400,000		700,000
Net impairment losses recognized in earnings				$ 100,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	347,249,000		347,249,000		195,176,000
Allowance for Expected Credit Loss	0		0
Gross Unrealized Gains	1,513,000		1,513,000		1,237,000
Gross Unrealized Losses	(15,000)		(15,000)		(210,000)
Fair Value	348,747,000		348,747,000		196,203,000
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,886,910,000		1,886,910,000		1,555,394,000
Allowance for Expected Credit Loss	0		0
Gross Unrealized Gains	146,562,000		146,562,000		99,328,000
Gross Unrealized Losses	(891,000)		(891,000)		(857,000)
Fair Value	2,032,581,000		2,032,581,000		1,653,865,000
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,627,745,000		2,627,745,000		2,711,910,000
Allowance for Expected Credit Loss	0		0
Gross Unrealized Gains	144,646,000		144,646,000		76,220,000
Gross Unrealized Losses	(2,748,000)		(2,748,000)		(3,008,000)
Fair Value	2,769,643,000		2,769,643,000		2,785,122,000
Asset backed securities (“ABS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	201,731,000		201,731,000		227,376,000
Allowance for Expected Credit Loss	(362,000)		(362,000)		0	$ 0
Gross Unrealized Gains	3,472,000		3,472,000		2,466,000
Gross Unrealized Losses	(145,000)		(145,000)		(178,000)
Fair Value	204,696,000		204,696,000		229,664,000
Residential mortgage backed securities (“RMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	398,934,000		398,934,000		271,384,000
Allowance for Expected Credit Loss	0		0
Gross Unrealized Gains	6,106,000		6,106,000		429,000
Gross Unrealized Losses	(1,147,000)		(1,147,000)		(3,227,000)
Fair Value	403,893,000		403,893,000		268,586,000
Commercial mortgage backed securities (“CMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	258,579,000		258,579,000		274,234,000
Allowance for Expected Credit Loss	0		0
Gross Unrealized Gains	13,791,000		13,791,000		5,531,000
Gross Unrealized Losses	(846,000)		(846,000)		(779,000)
Fair Value	271,524,000		271,524,000		278,986,000
Collateralized loan obligations (“CLOs”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	312,068,000		312,068,000		327,076,000
Allowance for Expected Credit Loss	0		0
Gross Unrealized Gains	0		0		33,000
Gross Unrealized Losses	(2,975,000)		(2,975,000)		(1,643,000)
Fair Value	309,093,000		309,093,000		$ 325,466,000
Debt securities issued by foreign sovereign governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,485,000		4,485,000
Allowance for Expected Credit Loss	0		0
Gross Unrealized Gains	56,000		56,000
Gross Unrealized Losses	0		0
Fair Value	$ 4,541,000		$ 4,541,000