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Share-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
Weighted-average grant-date fair values of stock options and the assumptions used in estimating the fair values are as follows:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Weighted-average grant-date fair value per share
$
20.10

 
$
10.36

 
$
19.04

Expected term (years)
5.4

 
5.2

 
5.6

Risk-free interest rate
2.00
%
 
1.66
%
 
1.70
%
Expected volatility
60.31
%
 
61.40
%
 
61.54
%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value
The following table summarizes our stock option activity:
 
December 31, 2017
 
Number
of Shares
 
Weighted-average
Exercise
Price
 
Aggregate Intrinsic Value
 
Weighted- average Remaining Contractual Term
Outstanding at beginning of year
1.11

 
$
31.13

 
 
 
 
Granted
0.32

 
37.06

 
 
 
 
Exercised
(0.08
)
 
25.71

 
 
 
 
Forfeited / Cancelled
(0.10
)
 
36.16

 
 
 
 
Outstanding at end of year
1.25

 
$
32.58

 
$
21.9

 
6.4 years
Vested and expected to vest at end of year
1.24

 
$
32.60

 
$
9.4

 
6.4 years
Exercisable at end of year
0.56

 
$
36.29

 
$
21.7

 
4.7 years
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]
The following table summarizes our unvested restricted stock and RSUs activity:
 
December 31, 2017
 
Number
of Shares
 
Weighted-Average
Grant-Date Fair Value
Unvested at beginning of year
0.33

 
$
24.06

Granted
0.16

 
37.13

Forfeited
(0.03
)
 
26.51

Vested
(0.13
)
 
28.52

Unvested at end of year
0.33

 
$
28.38
Performance units [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share-based Compensation, Activity [Table Text Block]
The following table, which is stated at a 100% earned percentage, summarizes our performance units activity:
 
December 31, 2017
 
Number
of Shares
 
Weighted-Average
Grant-Date Fair Value
Unvested at beginning of year
0.08

 
$
31.84

Granted
0.03

 
38.00

Forfeited / Cancelled
(0.01
)
 
31.14

Vested
(0.02
)
 
93.34

Unvested at end of year
0.08

 
$
24.81
Leveraged Restricted Share Awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share-based Compensation, Activity [Table Text Block]
The following table, which is stated at a 100% earned percentage, summarizes our leveraged restricted share unit awards activity:
 
December 31, 2017
 
Number
of Shares
 
Weighted-average
Grant-Date Fair Value
Unvested at beginning of year
0.010

 
$
106.90

Forfeited / Cancelled
(0.005
)
 
106.90

Vested
(0.005
)
 
106.90

Unvested at end of year
—

 
$
—