Investments - Sales of Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Proceeds received	$ 500,760	$ 429,251	$ 445,100
Proceeds received	50,113	21,210	31,621
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross gains realized	13,570	15,915	22,476
Gross losses realized	(11,842)	(4,163)	(5,487)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross gains realized	7,753	2,869	6,604
Gross losses realized	$ (1,972)	$ (935)	$ (672)