Investments - Realized and Unrealized Holding Gains (Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net realized gain on investments:
Total realized gains	$ 8,600	$ 69,700	$ 44,100
Total realized losses	(900)	(4,600)	(7,600)
Net realized gains on investments	7,700	65,100	36,500
Unrealized Gain (Loss) on Investments	(57,400)	0	0
Total net realized gain on investments	(49,700)	65,100	36,500
Change in unrealized holding gains (losses), net of tax:
Income tax (expense) benefit	10,100	(700)	3,000
Change in unrealized holding gains (losses) on investments, net of tax	(38,300)	3,200	(5,700)
Fixed maturities:
Net realized gain on investments:
Total realized gains	2,000	2,800	2,800
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	(48,400)	6,300	(14,400)
Fixed maturities: | OTTI
Net realized gain on investments:
Total realized losses	0	0	(2,300)
Equity securities:
Net realized gain on investments:
Total realized gains	6,600	66,700	29,200
Unrealized Gain (Loss) on Investments	(49,700)	0	0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	0	(13,000)	14,300
Equity securities: | Sales
Net realized gain on investments:
Total realized losses	(900)	(1,100)	(800)
Equity securities: | OTTI
Net realized gain on investments:
Total realized losses	0	(3,500)	(4,500)
Other Invested Assets [Member]
Net realized gain on investments:
Total realized gains	0	200	12,100
Unrealized Gain (Loss) on Investments	(7,700)	0	0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	$ 0	$ 10,600	$ (8,600)