Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014
Income Taxes
Reversal of valuation allowance against the company's deferred tax assets	$ 258.9
Valuation allowance against deferred tax assets	0	0
Net deferred tax assets	$ 185.9	91.8
Federal
Income Taxes
Period of NOL carryforward		20 years
Period of federal tax credit carryforwards		20 years
State | Minimum
Income Taxes
Period of NOL carryforward		10 years
State | Maximum
Income Taxes
Period of NOL carryforward		20 years