Investments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Other investments, at cost	$ 17,037	$ 17,455
Maximum percentage of shareholders' equity security holdings held of one issuer	10.00%	10.00%
Available-for-sale securities pledged as collateral, fair value	$ 315,800	$ 334,400
Available for sale securities transferred to held to maturity		46,600
Mortgage-backed securities, weighted average life	4 years
Des Moines
Schedule of Available-for-sale Securities [Line Items]
Other investments, at cost		$ 8,300