Interest rate hedge agreements Outstanding interest rate swap (Details) $ in Thousands	Mar. 31, 2018 USD ($) contract
Interest rate hedge agreements
Fair Values	$ 6,461
Notional Amount in Effect as of 3/31/18	600,000
Notional Amount in Effect as of 12/31/18	600,000
Notional Amount in Effect as of 12/31/19	$ 100,000
1.16% Interest rate swap, effective March 29, 2018
Interest rate hedge agreements
Number of Contracts | contract	8
Interest Pay Rate (as a percent)	1.16%
Fair Values	$ 5,813
Notional Amount in Effect as of 3/31/18	600,000
Notional Amount in Effect as of 12/31/18	600,000
Notional Amount in Effect as of 12/31/19	$ 0
1.89% Interest rate swap, effective March 29, 2019
Interest rate hedge agreements
Number of Contracts | contract	1
Interest Pay Rate (as a percent)	1.89%
Fair Values	$ 648
Notional Amount in Effect as of 3/31/18	0
Notional Amount in Effect as of 12/31/18	0
Notional Amount in Effect as of 12/31/19	$ 100,000