Investments - Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Total investment income	$ 386,707	$ 383,470	$ 370,753
Investment expenses	(10,200)	(9,840)	(9,567)
Net investment income	376,507	373,630	361,186
Fixed maturity securities
Net Investment Income [Line Items]
Total investment income	353,303	354,290	342,773
Equity securities
Net Investment Income [Line Items]
Total investment income	6,017	6,411	4,703
Limited partnership interests
Net Investment Income [Line Items]
Total investment income	15,406	12,555	13,609
Short-term and other investments
Net Investment Income [Line Items]
Total investment income	$ 11,981	$ 10,214	$ 9,668