Investments - Net Realized Investment Gains (Losses) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Issuer	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) debt_security	Dec. 31, 2013 USD ($)
Total other-than-temporary impairment losses:
OTTI losses	$ (10,834)	$ (19,917)	$ (1,475)
Portion of OTTI losses recognized in other comprehensive income before taxes	1,068	4,572	268
Net impairment losses recognized in earnings	(9,766)	(15,345)	(1,207)
Gross realized gains, available-for-sale securities	12,451	11,936	5,627
Gross realized (losses), available-for-sale securities	(7,038)	(11,481)	(1,103)
Net realized gains (losses), trading securities	6,632	1,080	28,018
Net realized gains (losses), Other investments	1,115	464	326
Change in unrealized holding gains (losses), trading securities	30,557	(28,343)	(18,883)
Change in unrealized holding gains (losses), convertible securities, carried at fair value	899	(896)	1,876
Other	25	946	0
Total net realized investment gains (losses)	34,875	(41,639)	14,654
Credit-related OTTI	1,158	5,751	$ 232	$ 83
Number of issuers | debt_security			2
State and municipal bonds
Total other-than-temporary impairment losses:
OTTI losses	(100)	0	$ (50)
Corporate debt
Total other-than-temporary impairment losses:
OTTI losses	(7,604)	(11,781)	(1,425)
Portion of OTTI losses recognized in other comprehensive income before taxes	900
Credit-related OTTI	$ 5,500	4,900
Number of issuers | Issuer	10
Other investments
Total other-than-temporary impairment losses:
OTTI losses	$ (3,130)	$ (8,136)	$ 0