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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
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:

 
2017
 
2016
 
2015
 
(in thousands)
Stock options
$
2,695

 
$
2,357

 
$
2,648

Restricted stock units
17,738

 
14,723

 
10,735

Unrestricted stock awards
974

 
955

 
706

Phantom stock units
1,747

 
1,077

 
—

Total stock-based compensation
$
23,154

 
$
19,112

 
$
14,089

 
 
 
 
 
 
Related tax benefit
$
5,034

 
$
4,927

 
$
4,228
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, 2015
1,123

 
$
51.90

 
4.4
 
$
1,676

 
 
Granted
291

 
35.25

 
 
 
 
 
$
12.09

Exercised
(24
)
 
36.05

 
 
 
26

 
 
Forfeited
(17
)
 
37.47

 
 
 
 
 
 
Expired
(193
)
 
52.17

 
 
 
 
 
 
Outstanding, December 31, 2015
1,180

 
$
48.31

 
5.7
 
$
405

 
 
 
 
 
 
 
 
 
 
 
 
Granted
191

 
$
40.40

 
 
 
 
 
$
13.27

Exercised
(58
)
 
37.00

 
 
 
$
742

 
 
Forfeited
(36
)
 
35.29

 
 
 
 
 
 
Expired
(318
)
 
55.13

 
 
 
 
 
 
Outstanding, December 31, 2016
959

 
$
45.64

 
6.6
 
$
19,125

 
 
 
 
 
 
 
 
 
 
 
 
Granted
135

 
$
65.94

 
 
 
 
 
$
21.99

Exercised
(41
)
 
39.92

 
 
 
$
1,071

 
 
Forfeited
(35
)
 
47.38

 
 
 
 
 
 
Expired
(62
)
 
70.12

 
 
 
 
 
 
Outstanding, December 31, 2017
956

 
$
47.10

 
6.3
 
$
21,965

 
 
 
 
 
 
 
 
 
 
 
 
Exercisable, December 31, 2017
640

 
$
46.08

 
5.3
 
$
15,934

 
 
 
 
 
 
 
 
 
 
 
 
Expected to vest, December 31, 2017
316

 
$
49.17

 
8.4
 
$
6,031

 
 
Stock Options, Valuation Assumptions [Table Text Block]
s
The weighted-average assumptions used to estimate the fair value of stock options granted and the resulting weighted average fair value are as follows:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Expected volatility
32.5
%
 
33.5
%
 
34.3
%
Risk-free interest rate
2.0
%
 
1.3
%
 
1.7
%
Expected term (years)
5.5

 
5.5

 
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, 2015
682

 
 
 
 
Granted
434

 
$
35.09

 
 
Released
(296
)
 
 
 
$
12,204

Forfeited
(64
)
 
 
 
 
Outstanding, December 31, 2015
756

 
 
 
 
 
 
 
 
 
 
Granted
306

 
$
41.58

 
 
Released
(312
)
 
 
 
$
11,944

Forfeited
(49
)
 
 
 
 
Outstanding, December 31, 2016
701

 
$
38.04

 
 
 
 
 
 
 
 
Granted
273

 
$
50.95

 
 
Released
(372
)
 
36.93

 
$
14,219

Forfeited
(46
)
 
48.56

 
 
Outstanding, December 31, 2017
556

 
$
47.68

 
 
 
 
 
 
 
 
Vested but not released, December 31, 2017
142

 
 
 
$
9,650

 
 
 
 
 
 
Expected to vest, December 31, 2017
350

 
 
 
$
23,877
Restricted Stock Units, Valuation Assumptions [Table Text Block]
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:

 
Year Ended December 31,
 
2017
 
2016
 
2015
Expected volatility
28.0
%
 
30.0
%
 
30.1
%
Risk-free interest rate
1.0
%
 
0.7
%
 
0.7
%
Expected term (years)
1.7

 
1.8

 
2.1

 
 
 
 
 
 
Weighted average grant date fair value
$
77.75

 
$
44.92

 
$
33.48
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, 2016
—

 
 
Granted
63

 
$
40.11

Forfeited
(1
)
 
 
Outstanding, December 31, 2016
62

 
 
 
 
 
 
Expected to vest, December 31, 2016
57

 
 
 
 
 
 
Outstanding, January 1, 2017
62

 
$
40.11

Granted
32

 
65.55

Released
(20
)
 
47.02

Forfeited
(11
)
 
40.11

Outstanding, December 31, 2017
63

 
$
47.28

 
 
 
 
Expected to vest, December 31, 2017
63