Share-Based Compensation - Restricted Stock Unit Activity (Details) $ / shares in Units, $ in Millions	12 Months Ended	97 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) shares
Time-based Restricted Stock Units
Units (In thousands)
Beginning balance (in shares)	1,028,256
Granted (in shares)	280,083
Vested (in shares)	(443,481)
Forfeited (in shares)	(150,795)
Ending balance (in shares)	714,063	1,028,256
Aggregate intrinsic value, nonvested | $	$ 25.0	$ 26.3
Shares vested, but not released (in shares)	615,643
Aggregate intrinsic value, shares vested but not released | $	$ 21.6
Total outstanding restricted stock units (in shares)	1,329,706
Aggregate intrinsic value, total outstanding restricted stock units | $	$ 46.6
Performance-Based Restricted Stock Units
Units (In thousands)
Beginning balance (in shares)	71,428
Granted (in shares)	0
Vested (in shares)	(14,286)
Forfeited (in shares)	0
Ending balance (in shares)	57,142	71,428
Aggregate intrinsic value, nonvested | $	$ 2.0	$ 1.8
Shares vested, but not released (in shares)	14,286
Aggregate intrinsic value, shares vested but not released | $	$ 0.5
Total outstanding restricted stock units (in shares)	71,428
Aggregate intrinsic value, total outstanding restricted stock units | $	$ 2.5
Market-Based Restricted Stock Units
Units (In thousands)
Beginning balance (in shares)	587,500
Granted (in shares)	243,168
Vested (in shares)	0
Forfeited (in shares)	(51,480)
Ending balance (in shares)	779,188	587,500
Aggregate intrinsic value, nonvested | $	$ 27.3	$ 15.0
Shares vested, but not released (in shares)	0
Aggregate intrinsic value, shares vested but not released | $	$ 0.0
Total outstanding restricted stock units (in shares)	779,188
Aggregate intrinsic value, total outstanding restricted stock units | $	$ 27.3
Former Director | Restricted Stock Units
Units (In thousands)
Vested (in shares)	(16,324)	(613,605)
Shares vested, but not released (in shares)	568,753
Vested awards, weighted average grant date fair value (in USD per share) | $ / shares	$ 9.29