Derivative Financial Instruments (Details) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Unrealized loss on forward starting swaps/Interest rate swaps	$ (6,732,000)		$ 2,187,000	$ (9,282,000)	$ 10,926,000
Expected net decrease to interest expense	(300,000)			(300,000)
Derivatives designated as cash flow hedges in prepaid expenses and other assets:
Interest rate swaps	0			0		$ 1,146,000
Derivatives designated as cash flow hedges in accounts payable, accrued expenses and other liabilities:
Interest rate swaps	284,000			284,000		3,581,000
Amount of unrealized gains/(losses) recognized in accumulated other comprehensive income/(loss) on derivatives:
Unrealized loss on forward starting swaps/Interest rate swaps	(6,732,000)		2,187,000	(9,282,000)	10,926,000
Amount of gains reclassified out of accumulated other comprehensive income/(loss) into interest expense:
Interest rate swaps	(283,000)		$ (654,000)	(1,158,000)	$ (1,275,000)
4.20% (4.234% effective rate) Notes due 2029 [Member]
Derivative [Line Items]
Principal debt amount		$ 350,000,000.0
Interest rate (in hundredths)		4.20%
Scheduled maturity date		Apr. 15, 2029
3.050% (3.079% effective rate) Notes due 2030 [Member]
Derivative [Line Items]
Principal debt amount	$ 400,000,000.0			$ 400,000,000.0
Interest rate (in hundredths)	3.05%			3.05%
Scheduled maturity date	Feb. 15, 2030
Forward Starting Swaps [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 150,000,000.0			$ 150,000,000.0		$ 225,000,000.0
Underlying treasury rate term (in periods)	10 years					10 years
Underlying treasury rate locked by forward-starting swaps (in hundredths)	1.87%			1.87%		2.86%
Unrealized loss on forward starting swaps/Interest rate swaps	$ (6,600,000)	$ (5,100,000)
Amount of unrealized gains/(losses) recognized in accumulated other comprehensive income/(loss) on derivatives:
Unrealized loss on forward starting swaps/Interest rate swaps	$ (6,600,000)	$ (5,100,000)