Liability for Unpaid Claims - Short-duration Contracts, Cumulative Paid Claims (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 573,008
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	24,707
Short-duration products	597,715
Short-duration Insurance Contracts, Accident Year 2013 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	83,103	$ 83,119	$ 83,131	$ 82,408	$ 68,159
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	0
Short-duration Insurance Contracts, Accident Year 2014 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	99,777	99,791	99,545	$ 81,054
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	0
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	141,242	140,982	$ 115,922
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	17
Short-duration Insurance Contracts, Accident Year 2016 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	138,468	$ 114,720
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	431
Short-duration Insurance Contracts, Accident Year 2017 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	110,418
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 24,259