Derivatives (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Mar. 07, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Unsecured notes		$ 683,970		$ 683,970		$ 708,734
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax		(9,227)	$ 4,289	(14,716)	$ 13,794
Amount reclassified from accumulated other comprehensive loss		584	1,415	408	3,553
Interest Expense		37,173	$ 38,074	74,925	$ 74,859
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months				5,300
Fair Value, Measurements, Recurring [Member]
Derivative [Line Items]
Interest Rate Cash Flow Hedge Asset at Fair Value		4,367		4,367		17,482
Interest Rate Cash Flow Hedge Liability at Fair Value		(1,619)		(1,619)		(5,491)
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivative [Line Items]
Interest Rate Cash Flow Hedge Asset at Fair Value		4,367		4,367		17,482
Interest Rate Cash Flow Hedge Liability at Fair Value		(1,619)		(1,619)		(5,491)
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net		2,748		$ 2,748		11,991
Treasury Lock [Member]
Derivative [Line Items]
Derivative, Inception Date				Dec. 06, 2018
Derivative, Maturity Date				Jun. 28, 2019
Derivative, Notional Amount		$ 250,000		$ 250,000
Derivative, Description of Variable Rate Basis				30 year U.S. Treasury
Derivative, Fixed Interest Rate	4.65%	3.147%		3.147%
Interest Rate Cash Flow Hedge Asset at Fair Value		$ 0		$ 0
Interest Rate Cash Flow Hedge Liability at Fair Value						(5,491)
Unsecured notes	$ 300,000
Unsecured notes maturity period	2049-03
Debt Instrument, Term	30 years
Treasury Lock [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Derivative [Line Items]
Unsecured notes	$ 5,700
Derivative $300M Term [Member]
Derivative [Line Items]
Derivative, Inception Date				Apr. 03, 2017
Derivative, Maturity Date				Dec. 02, 2020
Derivative, Notional Amount		$ 300,000		$ 300,000
Derivative, Description of Variable Rate Basis				1 Month LIBOR with Floor
Derivative, Fixed Interest Rate		1.824%		1.824%
Interest Rate Cash Flow Hedge Asset at Fair Value						3,759
Interest Rate Cash Flow Hedge Liability at Fair Value		$ (240)		$ (240)
Derivatives 200K @ 1.04770% and 65K @ 1.07000% [Member]
Derivative [Line Items]
Derivative, Inception Date				Aug. 01, 2016
Derivative, Maturity Date				Jan. 05, 2022
Derivative, Notional Amount		$ 265,000		$ 265,000
Derivative, Description of Variable Rate Basis				1 Month LIBOR with Floor
Derivative, Fixed Interest Rate		1.053%		1.053%
Interest Rate Cash Flow Hedge Asset at Fair Value		$ 3,975		$ 3,975		10,838
Derivative @ 1.30250% 20.0M [Member]
Derivative [Line Items]
Derivative, Inception Date				Apr. 07, 2016
Derivative, Maturity Date				Apr. 01, 2023
Derivative, Notional Amount		$ 19,942		$ 19,942
Derivative, Description of Variable Rate Basis				1 Month LIBOR
Derivative, Fixed Interest Rate		1.303%		1.303%
Interest Rate Cash Flow Hedge Asset at Fair Value		$ 217		$ 217		880
Derivative $33M Swap [Member]
Derivative [Line Items]
Derivative, Inception Date				Dec. 01, 2016
Derivative, Maturity Date				Nov. 01, 2023
Derivative, Notional Amount		$ 33,000		$ 33,000
Derivative, Description of Variable Rate Basis				1 Month LIBOR
Derivative, Fixed Interest Rate		1.49%		1.49%
Interest Rate Cash Flow Hedge Asset at Fair Value		$ 175		$ 175		1,376
Derivative @ 2.36600% 37.5K [Member]
Derivative [Line Items]
Derivative, Inception Date				Jun. 02, 2017
Derivative, Maturity Date				Jun. 02, 2027
Derivative, Notional Amount		$ 37,452		$ 37,452
Derivative, Description of Variable Rate Basis				1 Month LIBOR with Floor
Derivative, Fixed Interest Rate		2.366%		2.366%
Interest Rate Cash Flow Hedge Asset at Fair Value						$ 629
Interest Rate Cash Flow Hedge Liability at Fair Value		$ (1,379)		$ (1,379)
Swap [Member]
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months				$ 6,500