Summary of Significant Accounting Policies Awards Outstanding (Details) - shares shares in Thousands	Mar. 31, 2018	Dec. 31, 2017
Stock options
Awards outstanding [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	45	135
Restricted Stock Units (RSUs) [Member]
Awards outstanding [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	2,109	2,133
Restricted stock unit - market-based [Member]
Awards outstanding [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	345	345
Stock appreciation rights
Awards outstanding [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	303	356
Market Share Units (MSUs) [Member]
Awards outstanding [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	419	387