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Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details) - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 20, 2017 | Mar. 31, 2017 | Dec. 31, 2016
Share-based compensation arrangement by share-based payment award |  |  |
Shares issued, price per share |  |  | $ 742
Employee Stock Option | Combination One |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Non-option equity instruments, exercised |  |  | 5,000
Options, exercises in period, intrinsic value |  |  | $ 0.4
Shares issued, price per share |  |  | $ 742
Employee Stock Option | Combination Two |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Non-option equity instruments, exercised |  |  | 80,000
Options, exercises in period, intrinsic value |  |  | $ 8.4
Conversion of stock, shares converted |  |  | 9,930
Employee Stock Option | Chief Executive Officer |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Non-option equity instruments, exercised | 125,000 | 40,000 |
Options, exercises in period, intrinsic value |  | $ 4.4 |
Common Stock |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Conversion of stock, shares converted |  | 5,142 |

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- Definition Number of non-option equity instruments exercised by participants. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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