Income Taxes (Narrative) (Details) - USD ($)		3 Months Ended	12 Months Ended
	Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Discrete benefit due to new tax laws	$ (177,000,000.0)	$ (177,000,000.0)
Total unrecognized tax benefit		6,800,000	$ 6,800,000	$ 34,500,000	$ 35,600,000
Recognized interest expense			100,000	600,000	200,000
Interest accrued		200,000	200,000	200,000	1,700,000
Penalties accrued		0	0	0	$ 0
Cumulative valuation allowance against deferred tax assets		9,200,000	9,200,000	12,600,000
Net operating loss carryforwards		177,800,000	177,800,000
Net operating loss carryforwards corresponding to deferred tax assets		$ 42,300,000	$ 42,300,000	$ 31,700,000