Derivative Instruments - Estimated Fair Values of Derivative Instruments (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Interest rate swap agreement $440 million
Derivative Instruments, Gain (Loss)
Contract/Notional Amount	$ 440,000,000	$ 440,000,000
Carrying Amount	(2,768,000)	(1,475,000)
Fair Value	$ (2,768,000)	$ (1,475,000)
Interest rate swap agreement $440 million | LIBOR
Derivative Instruments, Gain (Loss)
Derivative, basis spread on variable rate (up to)	2.74%	2.74%
Interest rate swap agreement $200 million
Derivative Instruments, Gain (Loss)
Contract/Notional Amount	$ 200,000,000	$ 200,000,000
Carrying Amount	(1,259,000)	(663,000)
Fair Value	$ (1,259,000)	$ (663,000)
Interest rate swap agreement $200 million | LIBOR
Derivative Instruments, Gain (Loss)
Derivative, basis spread on variable rate (up to)	2.74%	2.74%
Interest rate cap agreement
Derivative Instruments, Gain (Loss)
Contract/Notional Amount	$ 0	$ 249,000,000
Carrying Amount	0	0
Fair Value	$ 0	0
Fair Value, Measurements, Recurring | Interest rate cap agreement
Derivative Instruments, Gain (Loss)
Fair Value		$ 0