Derivative Instruments and Hedging Activities - Cash Flow Hedges (Details) - Cash Flow Hedging - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2018		Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross notional amount outstanding		$ 2,170,000		$ 1,960,000
Derivative asset fair value in other assets		8,389	[1]	3,989	[2]
Derivative liability fair value in other liabilities	[1]	$ (18,876)		$ (10,219)
Weighted-average interest rate received		2.01%		1.58%
Weighted-average interest rate paid		1.97%		1.61%
Weighted-average maturity (in years)		2 years 2 days		2 years 2 months 29 days

[1] Certain cash flow hedges are transacted through a clearinghouse ("centrally cleared") and their change in fair value is settled by the counterparties to the transaction, which results in no fair value.

[2] This amount represents the fair value if credit risk related contingent features were triggered.