Investments - Net Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total other-than-temporary impairment losses:
OTTI losses	$ (13,200)	$ (10,834)	$ (19,917)
Portion of OTTI losses recognized in other comprehensive income before taxes	248	1,068	4,572
Net impairment losses recognized in earnings	(12,952)	(9,766)	(15,345)
Gross realized gains, available-for-sale securities	6,653	12,451	11,936
Gross realized (losses), available-for-sale securities	(3,123)	(7,038)	(11,481)
Net realized gains (losses), Short-term investments	(2)	18	(1)
Net realized gains (losses), trading securities	10,724	6,632	1,080
Net realized gains (losses), Other investments	2,963	1,115	464
Change in unrealized holding gains (losses), trading securities	11,243	30,557	(28,343)
Change in unrealized holding gains (losses), Other investments, carried at fair value	896	899	(896)
Other	7	7	947
Total net realized investment gains (losses)	16,409	34,875	(41,639)
Investment in unconsolidated subsidiaries
Total other-than-temporary impairment losses:
OTTI losses	(11,931)	0	0
Other investments
Total other-than-temporary impairment losses:
OTTI losses	0	(3,130)	(8,136)
State and municipal bonds
Total other-than-temporary impairment losses:
OTTI losses	(850)	(100)	0
Corporate debt
Total other-than-temporary impairment losses:
OTTI losses	(419)	(7,604)	(11,781)
Portion of OTTI losses recognized in other comprehensive income before taxes	$ 248	$ 1,068	$ 4,572