Derivative Financial Instruments (Details) - USD ($)	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Interest rate swaps	$ 2,187,000			$ (347,000)	$ 10,926,000	$ (31,000)
Expected increase to interest expense	2,500,000				2,500,000
Derivatives designated as cash flow hedges in prepaid expenses and other assets:
Interest rate swaps	4,773,000				4,773,000		$ 1,286,000
Amount of unrealized gains/(losses) recognized in accumulated other comprehensive income on derivatives (effective portion):
Interest rate swaps	2,187,000			(347,000)	10,926,000	(31,000)
Amount of (gains)/losses reclassified out of accumulated other comprehensive income into contractual interest expense (effective portion):
Interest rate swaps	$ (654,000)			$ 211,000	$ (1,275,000)	$ 992,000
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
Underlying treasury rate term (in periods)		10 years					10 years
Underlying treasury rate locked by forward-starting swaps (in hundredths)		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