Derivative Financial Instruments (Details) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jun. 30, 2019
Derivative [Line Items]
Interest rate swaps	$ 862,000		$ (136,000)	$ 8,739,000	$ 316,000
Derivatives designated as cash flow hedges in prepaid expenses and other assets:
Interest rate swaps	2,871,000			2,871,000		$ 1,286,000
Amount of unrealized gains/(losses) recognized in accumulated other comprehensive income on derivatives (effective portion):
Interest rate swaps	862,000		(136,000)	8,739,000	316,000
Amount of (gains)/losses reclassified out of accumulated other comprehensive income into contractual interest expense (effective portion):
Interest rate swaps	(515,000)		$ 297,000	(621,000)	$ 781,000
4.125% (4.271% effective rate) Notes due 2028 [Member]
Derivative [Line Items]
Principal debt amount		$ 350,000,000
Interest rate (in hundredths)		4.125%
Forward Starting Swaps [Member]
Derivative [Line Items]
Amount of borrowings, subject to swaps	$ 150,000,000.0			$ 150,000,000.0		$ 150,000,000
Underlying treasury rate term (in periods)	10 years					10 years
Underlying treasury rate locked by forward-starting swaps (in hundredths)	2.91%			2.91%		2.44%
Interest rate swaps		$ 7,000,000
Gain on hedge ineffectiveness in interest expense		200,000
Amount of unrealized gains/(losses) recognized in accumulated other comprehensive income on derivatives (effective portion):
Interest rate swaps		$ 7,000,000
Scenario, Forecast [Member]
Derivative [Line Items]
Expected increase to interest expense							$ 2,500,000