General (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Potentially dilutive securities (in shares)	68,000		0	68,000	0
Common stock dividends (in dollars per share)	$ 0.6300		$ 0.6275	$ 1.8900	$ 1.8825
Deferred policy acquisition cost amortization	$ 157,365,000		$ 150,929,000	$ 463,604,000	$ 447,971,000
Advertising expenses	10,800,000		12,500,000	31,700,000	36,100,000
Reinsurance retention policy, excess retention, amount reinsured					717,000,000
Reinsurance policy, amount retained				40,000,000
Ceded premiums earned	15,000,000		13,000,000	41,000,000	44,000,000
Policyholder benefits and claims incurred, ceded	(7,000,000)		1,000,000	(23,000,000)	(51,000,000)
Revenue from contract with customer	4,500,000		4,200,000	13,700,000	12,500,000
Cost of goods and services sold	2,800,000		2,500,000	9,000,000.0	7,800,000
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			4,400,000
Capitalized contract cost, accumulated amortization	(1,500,000)			(1,500,000)			(300,000)
Capitalized contract cost, amortization expense	600,000			1,200,000
Shareholders' equity	1,900,832,000		1,802,544,000	1,900,832,000	1,802,544,000		1,799,502,000
Uncollectible or written off accrued interest receivables		$ 0		0
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	$ 1,801,898,000		$ 1,703,861,000	$ 1,801,898,000	$ 1,703,861,000	$ 1,717,917,000	1,700,674,000	$ 1,669,315,000	$ 1,519,658,000
Retained Earnings [Member] | Cumulative Effect, Period Of Adoption, Adjustment [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Shareholders' equity						$ 0	$ (2,014,000)	$ 0	$ 0