Summary of Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands			3 Months Ended								12 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 22, 2017	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) segment $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Jan. 01, 2019 USD ($)
Accounting Policies [Line Items]
Operating lease liability			$ 33,998								$ 33,998
Investments
Maximum period considered for applying equity method accounting based on financial information provided by the investee											3 months
Accounts Receivable
Premiums receivable, allowance for doubtful accounts			578				$ 482				$ 578	$ 482
Deferred Policy Acquisition Costs
Amortization expense											147,945	146,601	$ 144,703
Marketing and Advertising Expense [Abstract]
Advertising costs											2,182	2,500	2,216
Earnings per Weighted Average Common Share
Net income from continuing operations			$ 28,102	$ 15,619	$ 25,934	$ 29,946	$ 18,346	$ 28,908	$ 26,816	$ 9,125	99,601	83,195	62,387
Allocation of income for participating shares											(523)	(496)	(392)
Net income from continuing operations attributed to common shareholders											$ 99,078	$ 82,699	$ 61,995
Total weighted average common shares outstanding, including participating shares | shares											15,281,363	15,170,754	15,105,558
Less: weighted average participating shares | shares											(80,231)	(90,485)	(94,807)
Basic earnings per share denominator | shares											15,201,132	15,080,269	15,010,751
Common equivalent shares- non-vested performance stock grants | shares											136,675	149,629	124,597
Diluted earnings per share denominator | shares											15,337,807	15,229,898	15,135,348
Basic earnings per share (in dollars per share) | $ / shares			$ 1.84	$ 1.02	$ 1.70	$ 1.97	$ 1.21	$ 1.90	$ 1.77	$ 0.60	$ 6.52	$ 5.48	$ 4.13
Diluted earnings per share (in dollars per share) | $ / shares			1.82	1.01	1.68	1.95	1.19	1.88	1.75	0.60	$ 6.46	$ 5.43	4.10
Antidilutive common shares | shares											0	0
Undistributed Earnings, Diluted [Abstract]
Basic earnings per share (in dollars per share) | $ / shares			1.84	1.02	1.70	1.97	1.21	1.90	1.77	0.60	$ 6.52	$ 5.48	4.13
Dividends declared | $ / shares											(3.40)	(3.20)	(3.00)
Undistributed earnings (loss) | $ / shares											3.12	2.28	1.13
Diluted earnings per share (in dollars per share) | $ / shares			$ 1.82	$ 1.01	$ 1.68	$ 1.95	$ 1.19	$ 1.88	$ 1.75	$ 0.60	6.46	5.43	4.10
Undistributed earnings (loss) - Diluted | $ / shares											$ 3.06	$ 2.23	$ 1.10
Premiums and Unearned Premiums
Term over which premiums are earned											1 year
Deferred revenue											$ 14,628	$ 14,649
Segments
Number of business segments | segment											1
Cumulative effect of adoption of updated accounting guidance for callable debt securities at January 1, 2019			$ 2,373								$ 2,373			$ (2,373)
Statutory tax rate		35.00%									21.00%
Minimum
Equipment and Leasehold Improvements
Estimated useful lives											3 years
Maximum
Equipment and Leasehold Improvements
Estimated useful lives											10 years
Accounting Standards Update 2017-03
Segments
Cumulative Effect of Adoption of ASU 2018-02 in Period of Adoption	$ (4,736)
Accounting Standards Update 2016-01
Segments
Cumulative effect of change in accounting principle	$ 16,895
Accounting Standards Update 2016-02 | Restatement Adjustment
Accounting Policies [Line Items]
Operating lease liability														$ 35,984