Share-Based Compensation - Restricted Stock Activity (Details) $ / shares in Units, $ in Millions	9 Months Ended	120 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) shares
Time-Based Restricted Stock Units
Number of Units
Beginning balance (in shares)	714,063
Granted (in shares)	194,981
Vested (in shares)	(272,971)	(615,643)
Forfeited (in shares)	(11,689)
Ending balance (in shares)	624,384	714,063
Nonvested shares, aggregate intrinsic value | $	$ 22.3	$ 25.0
Shares vested, but not released (in shares)	615,643
Shares vested, but not released, aggregate intrinsic value | $	$ 22.0
Total outstanding awards (in shares)	1,240,027
Total outstanding awards, aggregate intrinsic value | $	$ 44.3
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)	28,572
Shares vested, but not released, aggregate intrinsic value | $	$ 1.0
Total outstanding awards (in shares)	71,428
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)	(14,417)
Ending balance (in shares)	873,581	779,188
Nonvested shares, aggregate intrinsic value | $	$ 31.2	$ 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)	873,581
Total outstanding awards, aggregate intrinsic value | $	$ 31.2
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) | $ / shares	$ 9.38