Leases - Operating Lease Rental Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating lease rental income for leased assets
Operating lease rental income recognized	$ 8,300	$ 9,700	$ 9,400
Future minimum rental income under noncancelable operating leases with terms in excess of one year
2018	5,642
2019	5,642
2020	5,638
2021	5,564
2022	4,550
Thereafter	11,072
Total	$ 38,108