Derivative Instruments and Hedging Activities - Cash Flow Hedges (Details) - Cash Flow Hedging - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2018		Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross notional amount outstanding		$ 2,060,000		$ 1,960,000
Derivative asset fair value in other assets		7,291	[1]	3,989	[2]
Derivative liability fair value in other liabilities	[1]	$ (15,729)		$ (10,219)
Weighted-average interest rate received		1.78%		1.58%
Weighted-average interest rate paid		1.85%		1.61%
Weighted-average maturity (in years)		2 years 2 months		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.