Significant Accounting Policies - Recently Adopted Accounting Standards (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2019 USD ($) Obligations	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Performance obligations under subscriber's agreement | Obligations		2
Indemnity's management fee rate as a percent of direct and affiliated assumed written premiums of the Exchange (percentage)		25.00%
Deferred tax asset		$ 17,186			$ 24,101
Cumulative effect adjustment due to ASC 606		$ 2,377,558			$ 2,231,417
Cumulative effect adjustment due to ASU 2018-02	[1]		$ 0
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease assets				$ 32,700
Operating lease liability				$ 32,100
Accounting Standards Update 2014-09 | Adjustments due to ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liability						$ 48,500
Deferred tax asset						10,200
Cumulative effect adjustment due to ASC 606						$ (38,300)
Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment due to ASU 2018-02			$ 26,400

[1] A one-time adjustment was made in 2017 to reclassify stranded tax effects of the components of accumulated other comprehensive income ("AOCI") (loss) resulting from enactment of Tax Cuts and Jobs Act ("TCJA") from AOCI (loss) to retained earnings. See Note 2, "Significant Accounting Policies".