Significant Accounting Policies (Details) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018 USD ($) fund	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Obligations fund	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Revenue, Performance Obligation [Abstract]
Performance obligations under subscriber's agreement | Obligations										2
Indemnity's management fee rate as a percent of premiums written and assumed by the Exchange										25.00%
New Accounting Pronouncements
Decrease in accumulated other comprehensive income and increase in retained earnings due to ASU 2018-02	[1]										$ 0
Cumulative effect adjustment	[2]	$ 38,392								$ 38,392
Impact of adoption on our Statement of Financial Position and Operations
Deferred tax asset		24,101				$ 19,390				24,101	19,390		$ 29,578
Contract liability		51,727								51,727			48,514
Retained earnings		2,231,417				2,140,853				2,231,417	2,140,853		2,102,527
Operating revenue		576,468	$ 612,126	$ 621,458	$ 572,160	401,402	$ 442,492	$ 448,564	$ 399,316	2,382,212	1,691,774	$ 1,596,631
Operating expenses		$ 501,710	$ 515,431	$ 526,135	$ 494,593	342,777	$ 361,253	$ 365,116	$ 332,376	$ 2,037,869	1,401,522	1,303,114
Cash and cash equivalents
Cash and cash equivalents, maximum maturity at the date of purchase (in months)										3 months
Available-for-sale securities
Available-for-sale securities, maximum maturity to be classified as current assets (in months)										12 months
Limited partnerships
Number of real estate limited partnerships that do not meet the criteria of an investment company | fund		1								1
Agent bonus estimates
Performance period for agent bonuses (in months)										36 months
Period of prior year actual underwriting data used to estimate agent bonuses (in years)										2 years
Administrative services
Cash settlements											522,300	496,100
Software | Minimum
Fixed assets
Estimated useful life (in years)										3 years
Software | Maximum
Fixed assets
Estimated useful life (in years)										7 years
Equipment | Minimum
Fixed assets
Estimated useful life (in years)										3 years
Equipment | Maximum
Fixed assets
Estimated useful life (in years)										10 years
Buildings and building improvements | Minimum
Fixed assets
Estimated useful life (in years)										20 years
Buildings and building improvements | Maximum
Fixed assets
Estimated useful life (in years)										45 years
Less than 10%
Limited partnerships
General ownership percentage of limited partnerships (as a percent)		10.00%								10.00%
Balances without ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Deferred tax asset		$ 13,238				19,390				$ 13,238	19,390
Contract liability		0				0				0	0
Retained earnings		2,272,281				$ 2,140,853				2,272,281	2,140,853
Accounting Standards Update 2014-09 | Adjustments due to ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Deferred tax asset		10,863								10,863			10,188
Contract liability		51,727								51,727			48,514
Retained earnings		$ (40,864)								$ (40,864)			(38,326)
Accounting Standards Update 2018-02
New Accounting Pronouncements
Decrease in accumulated other comprehensive income and increase in retained earnings due to ASU 2018-02											(26,400)
Accounting Standards Update 2016-01
New Accounting Pronouncements
Cumulative effect adjustment													$ (100)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Period of time over which management fee revenue allocated to administrative services will be recognized (in years)		4 years								4 years
Management fee revenue, policy issuance and renewal services
Impact of adoption on our Statement of Financial Position and Operations
Management fee revenue, gross										$ 1,721,309
Less: change in allowance for management fee returned on cancelled policies										(1,742)
Operating revenue										1,719,567	1,662,625	1,567,431
Operating expenses										1,457,533	$ 1,401,522	$ 1,303,114
Management fee revenue, policy issuance and renewal services | Balances without ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Management fee revenue, gross										1,778,212
Less: change in allowance for management fee returned on cancelled policies										(1,800)
Operating revenue										1,776,412
Management fee revenue, policy issuance and renewal services | Accounting Standards Update 2014-09 | Adjustments due to ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Management fee revenue, gross										(56,903)
Less: change in allowance for management fee returned on cancelled policies										58
Operating revenue										(56,845)
Management fee revenue, administrative services
Impact of adoption on our Statement of Financial Position and Operations
Management fee revenue, gross										53,694
Less: change in allowance for management fee returned on cancelled policies										(62)
Operating revenue										53,632
Management fee revenue, administrative services | Balances without ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Management fee revenue, gross										0
Less: change in allowance for management fee returned on cancelled policies										0
Operating revenue										0
Management fee revenue, administrative services | Accounting Standards Update 2014-09 | Adjustments due to ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Management fee revenue, gross										53,694
Less: change in allowance for management fee returned on cancelled policies										(62)
Operating revenue										53,632
Administrative services reimbursements
Impact of adoption on our Statement of Financial Position and Operations
Operating revenue										580,336
Operating expenses										580,336
Administrative services reimbursements | Balances without ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Operating revenue										0
Operating expenses										0
Administrative services reimbursements | Accounting Standards Update 2014-09 | Adjustments due to ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Operating revenue										580,336
Operating expenses										580,336
Administrative services
Impact of adoption on our Statement of Financial Position and Operations
Operating revenue										633,968
Administrative services | Balances without ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Operating revenue										0
Administrative services | Accounting Standards Update 2014-09 | Adjustments due to ASC 606
Impact of adoption on our Statement of Financial Position and Operations
Operating revenue										$ 633,968

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

[2] Cumulative effect adjustments are primarily related to the implementation of new revenue recognition guidance effective January 1, 2018. See Note 2, "Significant Accounting Policies".