Stock-based Compensation - Stock options and SSARs outstanding (Details) (SSARs / Stock Options, USD $)	Dec. 31, 2014
SSARs / Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding at beginning of period (shares)	650,223
Exercisable at end of period (shares)	493,398
Vested or expected to vest in the future (shares)	649,954
Weighted average exercise price of shares outstanding at beginning of period (in US$ per share)	$ 18.12
Weighted average exercise price of shares exercisable at end of period (in US$ per share)	$ 18.39
Weighted average exercise price of shares vested or expected to vest in the future (in US$ per share)	$ 18.12