Stock-Based Compensation - Common Shares Reserved for Issuance (Details) - shares shares in Millions	Dec. 31, 2017	Dec. 31, 2016
Number of common shares reserved for issuance
Shares available for granting under the 2002 plan (in shares)	33.8	33.5
Options outstanding (in shares)	2.1	3.8
Total shares reserved for issuance (in shares)	35.9	37.3
Director Deferred Stock Ownership Plan
Number of common shares reserved for issuance
Total shares reserved for issuance (in shares)	0.1	0.1