Common Shareholder's Equity (Details) - shares	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
Common stock, shares authorized for issue (in shares)	480,000,000	480,000,000
Preferred stock , shares authorized for issue (in shares)	20,000,000	20,000,000
Number of shares reserved for issuance under share-based compensation plans, benefit plans, and dividend reinvestment plan (in shares)	4,600,000	4,600,000
Authorized shares of common stock available for issuance (in shares)	392,400,000	392,500,000
Authorized shares of preferred stock available for issuance (in shares)	20,000,000	20,000,000