Investments - Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Total investment income	$ 374,548	$ 386,707	$ 383,470
Investment expenses	(9,484)	(10,200)	(9,840)
Net investment income	365,064	376,507	373,630
Fixed maturity securities
Net Investment Income [Line Items]
Total investment income	283,228	353,303	354,290
Equity securities
Net Investment Income [Line Items]
Total investment income	4,923	6,017	6,411
Limited partnership interests
Net Investment Income [Line Items]
Total investment income	25,694	15,406	12,555
Short-term and other investments
Net Investment Income [Line Items]
Total investment income	$ 60,703	$ 11,981	$ 10,214