Income Taxes - Uncertain Tax Positions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits that would impact effective tax rate	$ 0
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance at the beginning of the year	3,188,000	$ 1,820,000	$ 2,608,000
Additions based on prior period tax positions	43,000	93,000	980,000
Additions based on current period tax positions	803,000	1,275,000	0
Reductions based on prior period tax positions	0	0	(1,768,000)
Balance at the end of the year	4,034,000	3,188,000	1,820,000
Interest related to unrecognized tax benefits	0	$ 0	$ 0
Tax Cuts and Jobs Act of 2017, incomplete accounting, change in tax rate, deferred tax liability, provisional income tax benefit	24,900,000
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for AMT credits, provisional income tax benefit	$ 1,700,000