Share-Based Compensation - Unvested Restricted Shares (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) $ / shares shares
Restricted Shares
Nonvested Restricted Shares
Unvested, beginning balance (in shares) | shares	35,630
Granted (in shares) | shares	169,781
Vested (in shares) | shares	(196,332)
Forfeited (in shares) | shares	0
Unvested, ending balance (in shares) | shares	9,079
Weighted Avg. Grant Date Fair Value
Unvested, beginning balance (in dollars per share) | $ / shares	$ 63.27
Granted (in dollars per share) | $ / shares	9.15
Vested (in dollars per share) | $ / shares	39.59
Forfeited (in dollars per share) | $ / shares	0
Unvested, ending balance (in dollars per share) | $ / shares	$ 57.92
Share-Settled RSUs
Nonvested Restricted Shares
Unvested, beginning balance (in shares) | shares	1,125,346
Granted (in shares) | shares	1,389,681
Vested (in shares) | shares	(280,394)
Forfeited (in shares) | shares	(238,283)
Unvested, ending balance (in shares) | shares	1,996,350
Weighted Avg. Grant Date Fair Value
Unvested, beginning balance (in dollars per share) | $ / shares	$ 37.91
Granted (in dollars per share) | $ / shares	7.47
Vested (in dollars per share) | $ / shares	41.47
Forfeited (in dollars per share) | $ / shares	34.37
Unvested, ending balance (in dollars per share) | $ / shares	$ 27.84
Unrecognized compensation costs | $	$ 32
Unrecognized compensation costs, not yet recognized	2 years
PSUs
Nonvested Restricted Shares
Unvested, beginning balance (in shares) | shares	806,233
Granted (in shares) | shares	0
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(213,843)
Unvested, ending balance (in shares) | shares	592,390
Weighted Avg. Grant Date Fair Value
Unvested, beginning balance (in dollars per share) | $ / shares	$ 34.12
Granted (in dollars per share) | $ / shares	0
Vested (in dollars per share) | $ / shares	0
Forfeited (in dollars per share) | $ / shares	26.74
Unvested, ending balance (in dollars per share) | $ / shares	$ 36.25