Derivatives - Location and Aggregate Fair Value of Interest Rate Swaps (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2018	Jul. 07, 2016
Derivative [Line Items]
Total notional amount			$ 250,000
Effective date	Jul. 07, 2016
Maturity date	Jun. 10, 2023
Interest Rate Swap | Carrying Amount | Recurring basis
Derivative [Line Items]
Asset balance (included in Other assets)		$ 14,693	$ 21,200
London Interbank Offered Rate (LIBOR) | 2018 Credit Agreement Amendment
Derivative [Line Items]
Reference rate (as a percent)	1.10%
London Interbank Offered Rate (LIBOR) | 2016 Credit Agreement Amendment
Derivative [Line Items]
Reference rate (as a percent)	1.20%
Unsecured Debt | London Interbank Offered Rate (LIBOR) | 2018 Credit Agreement Amendment
Derivative [Line Items]
Reference rate (as a percent)	1.25%	1.80%
Unsecured Debt | London Interbank Offered Rate (LIBOR) | 2016 Credit Agreement Amendment
Derivative [Line Items]
Reference rate (as a percent)		1.80%
Unsecured Debt | London Interbank Offered Rate (LIBOR) | Interest Rate Swap
Derivative [Line Items]
Effective fixed interest rate (as a percent)	1.07%	1.07%		2.32%	1.07%
Interest rate (as a percent)	2.32%	2.87%