Derivative Instruments and Hedging Activities - Cash Flow Hedges (Details) - Cash Flow Hedging - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross notional amount outstanding		$ 1,880,000	$ 2,280,000
Derivative asset fair value in other assets	[1]	2,059	6,889
Derivative liability fair value in other liabilities	[1]	$ (205)	$ (11,328)
Weighted-average interest rate received		2.02%	2.12%
Weighted-average interest rate paid		1.97%	2.20%
Weighted-average maturity (in years)		1 year 4 months 28 days	1 year 6 months 10 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.