Derivative Instruments and Hedging Activities - Offsetting Assets and Liabilities in the Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative assets
Gross amounts of recognized assets	$ 2,239	$ 2,991
Gross amounts offset in the statement of financial position	0	0
Net amounts of assets presented in the statement of financial position	2,239	2,991
Gross amounts of financial instruments not offset in the statement of financial position	0	0
Net amount	2,239	2,991
Derivative liabilities
Gross amounts of recognized liabilities	30,637	40,259
Gross amounts offset in the statement of financial position	0	0
Net amounts of liabilities presented in the statement of financial position	30,637	40,259
Gross amounts of financial instruments not offset in the statement of financial position	241	757
Net amount	$ 30,878	$ 41,016