Fair Value Measurements - Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Total Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 4,539	$ 4,959
Transfers out of Level 3	(3,100)
Total realized/unrealized gains (losses):
Included in earnings and reported as losses incurred, net	0	0
Purchases	0	0
Sales	(17)	(8)
Balance at end of period	1,422	4,951
Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	271	691
Transfers out of Level 3	0
Total realized/unrealized gains (losses):
Included in earnings and reported as losses incurred, net	0	0
Purchases	0	0
Sales	(17)	(8)
Balance at end of period	254	683
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	4,268	4,268
Transfers out of Level 3	(3,100)
Total realized/unrealized gains (losses):
Included in earnings and reported as losses incurred, net	0	0
Purchases	0	0
Sales	0	0
Balance at end of period	1,168	4,268
Real Estate Acquired
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	12,713	11,748
Transfers out of Level 3	0
Total realized/unrealized gains (losses):
Included in earnings and reported as losses incurred, net	341	(163)
Purchases	5,894	8,683
Sales	(8,870)	(9,538)
Balance at end of period	$ 10,078	$ 10,730