Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Nonvested Restricted Stock, Stock Grants and Stock Units

The following table is a summary of awards granted and the weighted-average grant date fair value for the three years ended 2017, 2016 and 2015.

	Restricted Stock Units, Stock Grants, and Stock Units (a)						Performance Shares (b)
	2017		2016		2015		2017		2016		2015
Units granted	161,963		141,811		152,651		147,706		166,666		151,430
Weighted-average grant date fair value	$	72.60	$	67.34	$	64.12	$	78.99	$	66.60	$	64.97

(a) Units granted includes awards that will be cash settled of 67,599 in 2017, 43,952 in 2016, and 45,104 in 2015.

(b) Reflects the target payout level.

The following table is a summary of the status of non-vested awards as of December 31, 2017 and changes during the year.

	Restricted Stock Units, Stock Grants, and Stock Units					Performance Shares
	Shares			Weighted-Average Grant Date Fair Value		Shares (b)		Weighted-Average Grant Date Fair Value
Nonvested at January 1, 2017	335,259			$	62.04	312,724		$	65.32
Granted	161,963			72.60		147,706		78.99
Change in performance factor	—			—		18,266		64.97
Vested	(202,327	)		59.19		(164,396	)	63.87
Forfeited (c)	(3,607	)		69.58		(4,798	)	69.77
Nonvested at December 31, 2017	291,288		(a)	69.78		309,502		72.46
Vested Awards Outstanding at December 31, 2017	89,928					164,396

(a) Includes 133,373 of awards that will be cash settled.

(b) The nonvested performance shares are reflected at target payout level.  The performance metric component increase or decrease in the number of shares from the target level to the estimated actual payout level is included in the increase for performance factor amounts in the year the award vests.

(c) We account for forfeitures as they occur.

Summary of Nonvested Performance Shares

The following table is a summary of the status of non-vested awards as of December 31, 2017 and changes during the year.

	Restricted Stock Units, Stock Grants, and Stock Units					Performance Shares
	Shares			Weighted-Average Grant Date Fair Value		Shares (b)		Weighted-Average Grant Date Fair Value
Nonvested at January 1, 2017	335,259			$	62.04	312,724		$	65.32
Granted	161,963			72.60		147,706		78.99
Change in performance factor	—			—		18,266		64.97
Vested	(202,327	)		59.19		(164,396	)	63.87
Forfeited (c)	(3,607	)		69.58		(4,798	)	69.77
Nonvested at December 31, 2017	291,288		(a)	69.78		309,502		72.46
Vested Awards Outstanding at December 31, 2017	89,928					164,396

(a) Includes 133,373 of awards that will be cash settled.

(b) The nonvested performance shares are reflected at target payout level.  The performance metric component increase or decrease in the number of shares from the target level to the estimated actual payout level is included in the increase for performance factor amounts in the year the award vests.

(c) We account for forfeitures as they occur.

The following table is a summary of awards granted and the weighted-average grant date fair value for the three years ended 2017, 2016 and 2015.

	Restricted Stock Units, Stock Grants, and Stock Units (a)						Performance Shares (b)
	2017		2016		2015		2017		2016		2015
Units granted	161,963		141,811		152,651		147,706		166,666		151,430
Weighted-average grant date fair value	$	72.60	$	67.34	$	64.12	$	78.99	$	66.60	$	64.97

(a) Units granted includes awards that will be cash settled of 67,599 in 2017, 43,952 in 2016, and 45,104 in 2015.

(b) Reflects the target payout level.