Derivative Instruments and Hedging Activities - Offsetting of Derivative Assets and Liabilities in the Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative assets
Gross amounts of recognized assets	$ 1,273	$ 2,239
Gross amounts offset in the statement of financial position	(25)	0
Net amounts of assets presented in the statement of financial position	1,248	2,239
Gross amounts of financial instruments not offset in the statement of financial position	0	0
Net amount	1,248	2,239
Derivative liabilities
Gross amounts of recognized liabilities	10,946	30,637
Gross amounts offset in the statement of financial position	(25)	0
Net amounts of liabilities presented in the statement of financial position	10,921	30,637
Gross amounts of financial instruments not offset in the statement of financial position	0	241
Net amount	$ 10,921	$ 30,878