Share-Based Compensation Plan (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Summary of Service Condition Restricted Share Activity

The following table provides a summary of service condition restricted share activity as of December 31, 2020:

	Shares	Weighted-Average Grant Date Fair Value
Unvested at January 1, 2018	137,105	$	30.05
Granted	52,609	$	36.86
Vested	(61,982)	$	31.35
Forfeited	—	$	—
Unvested at December 31, 2018	127,732	$	32.22
Granted	88,430	$	32.64
Vested	(66,276)	$	30.20
Forfeited	(707)	$	32.70
Unvested at December 31, 2019	149,179	$	33.37
Granted	390,242	$	23.62
Vested	(72,824)	$	33.13
Forfeited	(6,787)	$	27.68
Cancelled	(217,083)	$	25.53
Unvested at December 31, 2020	242,727	$	24.94

Performance-Based Equity Awards Methodology and Assumptions

The grant date fair value of the performance awards, with market conditions, were determined using a Monte Carlo simulation method with the following assumptions (dollars in millions):

Performance Award Grant Date		Percentage of Total Award	Grant Date Fair Value by Component ($ in millions)		Volatility	Interest Rate	Dividend Yield
December 13, 2013
	Relative Total Shareholder Return	50.00%	$4.7		29.00%	0.34% - 2.25%	2.40%
	Absolute Total Shareholder Return	50.00%	$2.9		29.00%	0.34% - 2.25%	2.40%
February 11, 2015
	Relative Total Shareholder Return	30.00%	$0.9		22.00%	1.02%	2.50%
	Absolute Total Shareholder Return	40.00%	$0.7		22.00%	1.02%	2.50%
	EBITDA Comparison	30.00%	$0.7		22.00%	1.02%	2.50%
July 27, 2015
	Relative Total Shareholder Return	30.00%	$—	(1)	22.00%	0.68%	2.50%
	Absolute Total Shareholder Return	40.00%	$—	(1)	22.00%	0.68%	2.50%
	EBITDA Comparison	30.00%	$—	(1)	22.00%	0.68%	2.50%
February 10, 2016
	Relative Total Shareholder Return	70.00%	$1.6		25.00%	0.71%	3.00%
	Absolute Total Shareholder Return	15.00%	$0.2		25.00%	0.71%	3.00%
	EBITDA Comparison	15.00%	$0.4		25.00%	0.71%	3.00%
February 15, 2017
	Relative and Absolute Total Shareholder Return	65.00% / 35.00%	$2.7		28.00%	1.27%	5.60%
February 14, 2018
	Relative and Absolute Total Shareholder Return	65.00% / 35.00%	$3.5		28.00%	2.37%	4.70%
February 13, 2019
	Relative and Absolute Total Shareholder Return	65.00% / 35.00%	$4.5		26.00%	2.52%	4.20%
February 12, 2020
	Relative Total Shareholder Return	100%	$4.9		23.40%	1.41%	—%

(1)Amounts round to zero.