Fair Value - Measured on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on NonRecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Accumulated amortization	$ 251,476	$ 139,546
Assets held for sale | Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on NonRecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning period	0	0
Assets transferred	0	0
Assets sold	0	0
Assets received	0	0
Balance, ending period	0	0
Assets included in discontinued operations and held for sale | Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on NonRecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning period	0	0
Assets transferred	0	0
Assets sold	0	0
Assets received	0	0
Balance, ending period	0	0
Other | Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on NonRecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning period	4,280	4,280
Assets transferred	0	0
Assets sold	0	0
Assets received	1,664	0
Balance, ending period	5,944	4,280
Accumulated amortization	$ 2,300	$ 1,600