General (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Condensed Balance Sheet Statements, Captions [Line Items]
Potentially dilutive securities (in shares)	0	67,500	0	67,500
Common stock dividends (in dollars per share)	$ 0.6325	$ 0.6300	$ 1.2650	$ 1.2600
Deferred policy acquisition cost amortization	$ 150,984,000	$ 149,706,000	$ 315,414,000	$ 306,240,000
Advertising expenses	11,600,000	9,200,000	21,400,000	20,800,000
Reinsurance reimbursable amount	31,000,000		$ 31,000,000
Reinsurance Retention Policy, Reinsured Risk, Percentage			71.00%
Reinsurance retention policy, excess retention, amount reinsured			$ 792,000,000
Reinsurance policy, amount retained			40,000,000
Ceded	16,000,000	12,000,000	31,000,000	26,000,000
Policyholder benefits and claims incurred, ceded	(4,000,000)	(15,000,000)	(3,000,000)	(16,000,000)
Revenue from contract with customer	5,000,000.0	4,500,000	11,300,000	9,200,000
Cost of goods and services sold	3,200,000	3,200,000	6,900,000	6,200,000
Contract with customer, asset	0		0			$ 0
Contract with customer, liability	0		0			0
Revenue, remaining performance obligation	0		0			0
Capitalized implementation costs for cloud computing	5,200,000		5,200,000			4,800,000
Capitalized contract cost, accumulated amortization	(3,400,000)		(3,400,000)			(2,100,000)
Capitalized contract cost, amortization expense	700,000	400,000	1,300,000	700,000
Shareholders' equity	2,178,631,000	1,816,815,000	2,178,631,000	1,816,815,000		2,032,597,000
Accounting Standards Update 2016-13 [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Increase (decrease) in accrued interest receivable, net			0
Retained Earnings [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Shareholders' equity	$ 2,079,759,000	$ 1,717,917,000	$ 2,079,759,000	$ 1,717,917,000	$ 2,005,600,000	1,933,627,000	$ 1,524,581,000	$ 1,700,674,000
Retained Earnings [Member] | Cumulative Effect, Period Of Adoption, Adjustment [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Shareholders' equity					$ 0	$ 0	$ 0	(2,014,000)
Retained Earnings [Member] | Cumulative Effect, Period Of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Shareholders' equity								$ 2,000,000