Derivative Instruments and Hedging Activities - Offsetting of Derivative Assets and Liabilities in the Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative Assets
Gross amounts of recognized assets	$ 48,445	$ 137,603
Gross amounts offset in the statement of financial position	0	0
Net amounts of assets presented in the statement of financial position	48,445	137,603
Gross amounts of financial instruments not offset in the statement of financial position	0	2,338
Net amount	$ 48,445	$ 139,941