Share-Based Payments - Nonvested Shares of Time-Based or Performance-Based Vesting Schedule (Details) - shares shares in Thousands	Dec. 31, 2017	Dec. 31, 2016
Time-based restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested shares (in shares)	692
Performance-based restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested shares (in shares)	1,303
Restricted stock awards and restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested shares (in shares)	1,995	1,992