Investments - Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Total investment income	$ 383,470	$ 370,753	$ 341,733
Investment expenses	(9,840)	(9,567)	(9,133)
Net investment income	373,630	361,186	332,600
Fixed maturity securities
Net Investment Income [Line Items]
Total investment income	354,290	342,773	326,207
Equity securities
Net Investment Income [Line Items]
Total investment income	6,411	4,703	4,355
Short-term and other investments
Net Investment Income [Line Items]
Total investment income	10,214	9,668	9,187
Other invested assets (equity method investments)
Net Investment Income [Line Items]
Total investment income	$ 12,555	$ 13,609	$ 1,984