Fair Value Measurements (Details) - Derivative Instruments - Interest Rate Swap $ in Thousands	Jun. 30, 2018 USD ($) derivative	Dec. 31, 2017 USD ($) derivative
Designated as Hedging Instrument
Fair Value, (Assets) Liabilities Measured on Recurring Basis [Line Items]
Number of interest rate swap agreements | derivative	5	5
Derivative, notional amount	$ 570,000	$ 570,000
Not Designated as Hedging Instrument | Mortgages
Fair Value, (Assets) Liabilities Measured on Recurring Basis [Line Items]
Number of interest rate swap agreements | derivative	2
Fair Value, Inputs, Level 2
Fair Value, (Assets) Liabilities Measured on Recurring Basis [Line Items]
Interest rate derivative liabilities, at fair value	$ 16	340
Fair Value, Inputs, Level 2 | Designated as Hedging Instrument
Fair Value, (Assets) Liabilities Measured on Recurring Basis [Line Items]
Interest rate derivative assets, at fair value	14,337	6,544
Interest rate derivative liabilities, at fair value	0	85
Fair Value, Inputs, Level 2 | Not Designated as Hedging Instrument | Mortgages
Fair Value, (Assets) Liabilities Measured on Recurring Basis [Line Items]
Interest rate derivative liabilities, at fair value	$ 16	$ 255