Income Taxes - Net Operating Losses and Tax Rates (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Net operating losses
Statutory Federal income tax rate (as a percent)	35.00%
Alternative minimum tax credit without expiration	$ 23,443	$ 17,589
Tax credit recovery period	4 years
Federal
Net operating losses
Operating loss carryforward	$ 491,400	763,900
Estimated effective tax rate on net operating losses	3.36
State
Net operating losses
Operating loss carryforward	$ 302,500	$ 469,200
Estimated effective tax rate on net operating losses	21.0