Investments - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Other investments, at cost	$ 17,037,000	$ 12,605,000
Maximum percentage of shareholders' equity security holdings held of one issuer	10.00%	10.00%
Available-for-sale securities pledged as collateral, fair value	315,800,000	270,100,000
Available for sale securities transferred to held to maturity	46,600,000
Mortgage-backed securities, weighted average life	4 years
Des Moines
Schedule of Available-for-sale Securities [Line Items]
Other investments, at cost	9,600,000
San Francisco
Schedule of Available-for-sale Securities [Line Items]
Other investments, at cost	$ 300,000