Derivative Financial Instruments (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Mar. 31, 2019
Derivatives designated as cash flow hedges in prepaid expenses and other assets:
Interest rate swaps	$ 2,179,000		$ 1,286,000
Amount of unrealized gains recognized in accumulated other comprehensive income on derivatives (effective portion):
Interest rate swaps	7,877,000	$ 452,000
Amount of (gains)/losses reclassified out of accumulated other comprehensive income into contractual interest expense (effective portion):
Interest rate swaps	(106,000)	$ 484,000
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
Underlying treasury rate term (in periods)			10 years
Underlying treasury rate locked by forward-starting swaps (in hundredths)			2.44%
Gain on forward-starting interest rate swaps	$ 7,000,000
Gain on hedge ineffectiveness in interest expense	$ 200,000
Scenario, Forecast [Member]
Derivative [Line Items]
Expected increase to interest expense				$ 2,400,000