Liability for Unpaid Claims - Short-duration Contracts, Claims Development (Details) claim in Thousands, $ in Thousands	Dec. 31, 2017 USD ($) claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 597,715
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	24,707
Short-duration Insurance Contracts, Accident Year 2013 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	83,103	$ 83,119	$ 83,151	$ 82,644	$ 84,111
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Number of Reported Claims | claim	1,337
Short-duration Insurance Contracts, Accident Year 2014 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 99,777	99,810	99,876	$ 101,407
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Number of Reported Claims | claim	1,600
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 141,259	141,460	$ 141,667
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 17
Short-duration Insurance Contracts, Number of Reported Claims | claim	2,224
Short-duration Insurance Contracts, Accident Year 2016 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 138,899	$ 140,944
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 431
Short-duration Insurance Contracts, Number of Reported Claims | claim	2,158
Short-duration Insurance Contracts, Accident Year 2017 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 134,677
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 24,259
Short-duration Insurance Contracts, Number of Reported Claims | claim	1,765