COMMON STOCK Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Schedule of Stock by Class [Table Text Block]

The following table provides a summary of our public offerings of common stock in 2016 and 2015:

Date	Shares Issued	Price per Share		Net Proceeds
				(in millions)
September 2016	9,085,000	$	61.51	$	558.5
March 2016	5,922,500	50.11		296.6
March 2015	4,002,000	50.73		202.9

Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block]

The following table provides a summary of the impact of our outstanding stock-based compensation plans on the results of operations for the periods presented:

	Year Ended December 31,
	2017			2016			2015
	(in thousands)
Stock-based compensation expense	$	19,353		$	19,502		$	20,068
Income tax benefit	(7,372		)	(7,296		)	(7,636		)
Net stock-based compensation expense	$	11,981		$	12,206		$	12,432

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

he Compensation Committee has awarded SARs to our executive officers in 2017, 2016, and 2015. The fair value of each SAR award was estimated on the date of grant using a Black-Scholes pricing model using the following assumptions:

	Year Ended December 31,
	2017			2016			2015
Expected term of award (in years)	6.0 years			6.0 years			5.2 years
Risk-free interest rate	2.0		%	1.8		%	1.4		%
Expected volatility	53.3		%	54.5		%	58.0		%
Weighted-average grant date fair value per share	$	38.58		$	26.96		$	22.23

Schedule of Share-based Compensation, Stock Appreciation Rights Award Activity [Table Text Block]

The following table presents the changes in our SARs for all periods presented (in thousands, except per share data):

	Year Ended December 31,
	2017						2016						2015
	Number of SARs	Weighted-Average Exercise Price		Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value		Number of SARs		Weighted-Average Exercise Price		Aggregate Intrinsic Value		Number of SARs		Weighted-Average Exercise Price		Aggregate Intrinsic Value
Outstanding at January 1,	244,078	$	41.36	6.9	$	7,620	326,453		$	38.99	$	4,697	279,011		$	38.77	$	1,472
Awarded	54,142	74.57		—	—		58,709		51.63		—		68,274		39.63		—
Exercised	—	—		—	—		(141,084	)	40.16		2,770		(20,832	)	38.05		473
Outstanding at December 31	298,220	47.39		6.5	2,490		244,078		41.36		7,620		326,453		38.99		4,697
Exercisable at December 31	223,865	43.28		5.9	2,267		174,919		38.72		5,924		222,489		37.70		3,489

Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]

The following table presents the changes in non-vested time-based RSUs during 2017:

	Shares		Weighted-Average Grant Date Fair Value per Share
Non-vested at December 31, 2016	479,642		$	56.09
Granted	273,941		65.14
Vested	(266,809	)	57.67
Forfeited	(14,642	)	62.92
Non-vested at December 31, 2017	472,132		60.23

The following table presents the weighted-average grant date fair value per share and related information as of/for the periods presented:

	As of/Year Ended December 31,
	2017		2016		2015
	(in thousands, except per share data)
Total intrinsic value of time-based awards vested	$	16,303	$	18,973	$	17,077
Total intrinsic value of time-based awards non-vested	24,334		34,812		28,029
Market price per common share as of December 31,	51.54		72.58		53.38
Weighted-average grant date fair value per share	65.14		58.52		48.88

Restricted Stock Awards, Market-Based, Valuation assumptions [Table Text Block]

he weighted-average grant date fair value per PSU granted was computed using the Monte Carlo pricing model using the following assumptions:

	Year Ended December 31,
	2017			2016			2015
Expected term of award (in years)	3 years			3 years			3 years
Risk-free interest rate	1.4		%	1.2		%	0.9		%
Expected volatility	51.4		%	52.3		%	53.0		%
Weighted-average grant date fair value per share	$	94.02		$	72.54		$	66.16

Schedule of Nonvested Performance-based Units Activity [Table Text Block]

The following table presents the change in non-vested market-based awards during 2017:

	Shares		Weighted-Average Grant Date Fair Value per Share
Non-vested at December 31, 2016	48,420		$	64.97
Granted	28,069		94.02
Vested	(24,140	)	57.35
Non-vested at December 31, 2017	52,349		84.06

The following table presents the weighted-average grant date fair value per share and related information as of/for the periods presented:

	As of/Year Ended December 31,
	2017		2016		2015
	(in thousands, except per share data)
Total intrinsic value of market-based awards vested	$	2,687	$	6,562	$	4,293
Total intrinsic value of market-based awards non-vested	2,698		3,514		3,819
Market price per common share as of December 31,	51.54		72.58		53.38
Weighted-average grant date fair value per share	94.02		72.54		66.16