Income Taxes - Net Operating Losses and Tax Rates (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net operating losses
Statutory Federal income tax rate (as a percent)	21.00%	35.00%	35.00%
Alternative minimum tax credits	$ 15,744	$ 23,443
Tax credit recovery period	3 years
Federal
Net operating losses
Operating loss carryforward	$ 1,504,900	491,400
Estimated effective tax rate on net operating losses	21.00%
State
Net operating losses
Operating loss carryforward	$ 562,000	$ 302,500
Estimated effective tax rate on net operating losses	3.36%