Fair Value Measurements - Unobservable Input Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 4,959	$ 4,083	$ 2,167
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as net realized investment gains		3,579
Included in earnings and reported as losses incurred, net	0	0	0
Purchases	0	4,258	2,541
Sales	(420)	(6,961)	(625)
Balance at end of period	4,539	4,959	4,083
Total debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	691	1,228	1,846
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as net realized investment gains		0
Included in earnings and reported as losses incurred, net	0	0	0
Purchases	0	0	7
Sales	(420)	(537)	(625)
Balance at end of period	271	691	1,228
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	4,268	2,855	321
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as net realized investment gains		3,579
Included in earnings and reported as losses incurred, net	0	0	0
Purchases	0	4,258	2,534
Sales	0	(6,424)	0
Balance at end of period	4,268	4,268	2,855
Real Estate Acquired
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	11,748	12,149	12,658
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as net realized investment gains		0
Included in earnings and reported as losses incurred, net	(1,315)	(1,142)	(2,322)
Purchases	34,749	36,859	34,624
Sales	(32,469)	(36,118)	(32,811)
Balance at end of period	$ 12,713	$ 11,748	$ 12,149