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Share-based Compensation Plans (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs

Stock-based compensation expense, which is included in Operations and maintenance on the accompanying Consolidated Statements of Income, was as follows for the years ended December 31 (in thousands):

			2020			2019			2018
Stock-based compensation expense			$	5,373		$	12,095		$	12,390

Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity

A summary of the status of the restricted stock and restricted stock units at December 31, 2020, was as follows:

			Restricted Stock			Weighted-Average Grant Date Fair Value
			(in thousands)
Balance at January 1, 2020			192			$	65.66
Granted			116			69.49
Vested			(90)			63.30
Forfeited			(22)			65.30
Balance at December 31, 2020			196			$	69.05

The weighted-average grant-date fair value of restricted stock granted and the total fair value of shares vested during the years ended December 31, were as follows:

			Weighted-Average Grant Date Fair Value			Total Fair Value of Shares Vested
						(in thousands)
2020			$	69.49		$	6,722
2019			$	73.66		$	8,438
2018			$	57.31		$	6,776

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award

Outstanding performance periods at December 31, 2020 were as follows (shares in thousands):

									Possible Payout Range of Target
Grant Date			Performance Period			Target Grant of Shares			Minimum			Maximum
January 1, 2020			January 1, 2020 - December 31, 2022			36			0%			200%
January 1, 2019			January 1, 2019 - December 31, 2021			36			0%			200%
January 1, 2018			January 1, 2018 - December 31, 2020			49			0%			200%

A summary of the status of the Performance Share Plan at December 31, 2020 was as follows:

			Equity Portion						Liability Portion
						Weighted-Average Grant Date Fair Value (a)						Weighted-Average Fair Value at
			Shares						Shares			December 31, 2020
			(in thousands)						(in thousands)
Performance Shares balance at beginning of period			67			$	64.32		67
Granted			19			81.42			19
Forfeited			(2)			73.89			(2)
Vested			(23)			63.52			(23)
Performance Shares balance at end of period			61			$	69.71		61			$	52.42

_____________________

(a)    The grant date fair values for the performance shares granted in 2020, 2019 and 2018 were determined by Monte Carlo simulation using a blended volatility of 18%, 21% and 21%, respectively, comprised of 50% historical volatility and 50% implied volatility and the average risk-free interest rate of the three-year United States Treasury security rate in effect as of the grant date.

The weighted-average grant-date fair value of performance share awards granted was as follows in the years ended:

			Weighted Average Grant Date Fair Value
December 31, 2020			$	81.42
December 31, 2019			$	68.72
December 31, 2018			$	61.82

Performance Plan Payouts

Performance plan payouts have been as follows (in thousands):

Performance Period			Year Paid			Stock Issued			Cash Paid			Total Intrinsic Value
January 1, 2017 to December 31, 2019			2020			14			$	1,100		$	2,199
January 1, 2016 to December 31, 2018			2019			44			$	2,860		$	5,720
January 1, 2015 to December 31, 2017			2018			—			—			—

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