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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2018
Share-based Compensation [Abstract]  
Stock-Based Compensation Expense and Related Tax Benefit [Table Text Block]
Total stock-based compensation expense and the related tax benefit were as follows:
 
Three Months Ended March 31,
 
2018
 
2017
 
(in thousands)
Stock options
$
831

 
$
659

Restricted stock units
7,057

 
4,297

Unrestricted stock awards
207

 
255

Phantom stock units
690

 
392

Total stock-based compensation
$
8,785

 
$
5,603

 
 
 
 
Related tax benefit
$
1,534

 
$
1,228
Employee Stock Options Activity [Table Text Block]
A summary of our stock option activity is as follows:
 
Shares
 
Weighted
Average Exercise
Price per Share
 
Weighted Average
Remaining
Contractual Life
 
Aggregate
Intrinsic Value
 
Weighted
Average Grant
Date Fair Value
 
(in thousands)
 
 
 
(years)
 
(in thousands)
 
 
Outstanding, January 1, 2017
959

 
$
45.64

 
6.6
 
$
19,125

 
 
Granted
121

 
65.55

 
 
 
 
 
$
22.01

Exercised
(5
)
 
35.29

 
 
 
120

 
 
Outstanding, March 31, 2017
1,075

 
$
47.92

 
6.7
 
$
17,236

 
 
 
 
 
 
 
 
 
 
 
 
Outstanding, January 1, 2018
956

 
$
47.10

 
6.3
 
$
21,965

 
 
Converted upon acquisition
42

 
51.86

 
 
 
 
 
$
14.86

Granted
101

 
69.30

 
 
 
 
 
$
24.83

Exercised
(62
)
 
40.31

 
 
 
2,104

 
 
Forfeited
(3
)
 
72.25

 
 
 
 
 
 
Expired
(7
)
 
95.96

 
 
 
 
 
 
Outstanding, March 31, 2018
1,027

 
$
49.49

 
6.5
 
$
24,051

 
 
 
 
 
 
 
 
 
 
 
 
Exercisable March 31, 2018
716

 
$
45.96

 
5.5
 
$
19,725

 
 
 
 
 
 
 
 
 
 
 
 
Expected to vest, March 31, 2018
308

 
$
57.74

 
8.9
 
$
4,255

 
 
Stock Options, Valuation Assumptions [Table Text Block]
The weighted-average assumptions used to estimate the fair value of stock options granted and the resulting weighted average fair value are as follows:
 
Three Months Ended March 31,
 
2018
 
2017
Expected volatility
30.9
%
 
32.7
%
Risk-free interest rate
2.8
%
 
2.0
%
Expected term (years)
6.1

 
5.5
Restricted Stock Units Award Activity [Table Text Block]
The following table summarizes restricted stock unit activity:
 
Number of
Restricted Stock Units
 
Weighted
Average Grant
Date Fair Value
 
Aggregate
Intrinsic Value
 
(in thousands)
 
 
 
(in thousands)
Outstanding, January 1, 2017
701

 

 
 
Granted
131

 
$
63.12

 
 
Released
(317
)
 

 
$
12,066

Forfeited
(3
)
 

 
 
Outstanding, March 31, 2017
512

 

 
 
 
 
 
 
 
 
Outstanding, January 1, 2018
556

 
$
47.68

 
 
Converted upon acquisition
579

 
69.40

 
 
Granted
136

 
69.30

 
 
Released
(352
)
 
47.18

 
$
17,231

Forfeited
(28
)
 
68.44

 
 
Outstanding, March 31, 2018
891

 
63.72

 
 
 
 
 
 
 
 
Vested but not released, March 31, 2018
10

 
 
 
$
729

 
 
 
 
 
 
Expected to vest, March 31, 2018
788

 
 
 
$
56,369
Restricted Stock Units, Valuation Assumptions
The weighted-average assumptions used to estimate the fair value of performance-based restricted stock units granted and the resulting weighted average fair value are as follows:
 
Three Months Ended March 31,
 
2018
 
2017
Expected volatility
28.0
%
 
28.0
%
Risk-free interest rate
2.2
%
 
1.0
%
Expected term (years)
2.1

 
1.7

 
 
 
 
Weighted average fair value
$
78.56

 
$
77.78
Schedule of Other Share-based Compensation, Activity [Table Text Block]
The following table summarizes phantom stock unit activity:
 
Number of Phantom Stock Units
 
Weighted
Average Grant
Date Fair Value
 
(in thousands)
 
 
Outstanding, January 1, 2017
62

 


Granted
32

 
$
65.55

Released
(19
)
 


Forfeited
(2
)
 


Outstanding, March 31, 2017
73

 


 
 
 
 
Expected to vest, March 31, 2017
73

 
 
 
 
 
 
Outstanding, January 1, 2018
63

 
$
51.88

Converted upon acquisition
21

 
69.40

Granted
31

 
69.30

Released
(27
)
 
50.19

Forfeited
(1
)
 
69.36

Outstanding, March 31, 2018
87

 
62.61

 
 
 
 
Expected to vest, March 31, 2018
85