Share-Based Compensation - Restricted Stock Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	120 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Time-Based Restricted Stock Units
Number of Units
Beginning balance (in shares)	714,063
Granted (in shares)	174,661
Vested (in shares)	(243,009)	(615,643)
Forfeited (in shares)	(2,283)
Ending balance (in shares)	643,432	714,063
Nonvested shares, aggregate intrinsic value	$ 21.9	$ 25.0
Shares vested, but not released (in shares)	615,643
Shares vested, but not released, aggregate intrinsic value	$ 21.0
Total outstanding awards (in shares)	1,259,075
Total outstanding awards, aggregate intrinsic value	$ 42.9
Performance-Based Restricted Stock Units
Number of Units
Beginning balance (in shares)	57,142
Granted (in shares)	0
Vested (in shares)	(14,286)	(14,286)
Forfeited (in shares)	0
Ending balance (in shares)	42,856	57,142
Nonvested shares, aggregate intrinsic value	$ 1.5	$ 2.0
Shares vested, but not released (in shares)	28,571
Shares vested, but not released, aggregate intrinsic value	$ 1.0
Total outstanding awards (in shares)	71,427
Total outstanding awards, aggregate intrinsic value	$ 2.5
Market-Based Restricted Stock Units
Number of Units
Beginning balance (in shares)	779,188
Granted (in shares)	108,810
Vested (in shares)	0
Forfeited (in shares)	(6,186)
Ending balance (in shares)	881,812	779,188
Nonvested shares, aggregate intrinsic value	$ 30.0	$ 27.3
Shares vested, but not released (in shares)	0
Shares vested, but not released, aggregate intrinsic value	$ 0.0
Total outstanding awards (in shares)	881,812
Total outstanding awards, aggregate intrinsic value	$ 30.0
Resigned Trustee | Restricted Stock Units
Number of Units
Vested (in shares)	(14,286)
Shares vested, but not released (in shares)	568,753
Weighted average grant date fair value of vested awards (in USD per share)	$ 9.38