Derivative Instruments - Balance Sheet Location - (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Offsetting Derivative Assets
Gross amounts recognized	$ 0	$ 5
Gross amounts offset in the consolidated balance sheets	0	0
Net amounts presented in the consolidated balance sheets	0	5
Financial instruments	0	5
Financial collateral pledged	0	0
Net amounts	0	0
Offsetting Derivative Liabilities
Gross amounts recognized	11,214	5,182
Gross amounts offset in the consolidated balance sheets	0	0
Net amounts presented in the consolidated balance sheets	11,214	5,182
Financial instruments	0	5
Financial collateral pledged	7,747	4,879
Net amounts	$ 3,467	$ 298