Description of Business and Significant Accounting Policies Description of Business and Significant Accounting Policies - Recent Accounting Pronouncements (Details) $ in Millions	Dec. 31, 2016 USD ($)
Internal Revenue Service (IRS) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax deductions in excess of previously recorded stock-based compensation windfall tax benefits but not reflected in deferred tax assets	$ 39