Investments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Other investments, at cost	$ 26,661	$ 14,840
Maximum percentage of shareholders' equity security holdings held of one issuer	10.00%
Available-for-sale securities pledged as collateral, fair value	$ 500,000	407,300
Mortgage-backed securities, weighted average life	4 years
Des Moines
Schedule of Available-for-sale Securities [Line Items]
Other investments, at cost	$ 12,900	$ 4,400