Derivative Instruments and Hedging Activities - Interest Rate Swap Agreements (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Unrealized gain (loss) on cash flow hedges	$ (1,496,000)	$ 1,980,000	$ (9,101,000)	$ 4,183,000
Interest rate swap agreements
Derivative [Line Items]
Notional amount	350,000,000		350,000,000		$ 350,000,000	$ 350,000,000
Unrealized gain (loss) on cash flow hedges	(3,468,000)	$ 1,980,000	(11,073,000)	$ 4,183,000
Cumulative net losses in AOCI	12,046,000		12,046,000
Other Long-Term Liabilities | Interest rate swap agreements
Derivative [Line Items]
Derivative liability	$ 12,046,000		$ 12,046,000			$ 973,000