Summary of Significant Accounting Policies Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss)		$ 275.8	$ 202.8	$ 165.5	$ 412.2	$ (106.5)	$ 275.2	$ 197.7	$ 4.0	$ 1,056.4		$ 370.5		$ 560.5
Net Income (Loss) Available to Common Stockholders, Basic										1,056.4		370.5		560.5
Adjustment for interest expense incurred on assumed conversions of convertible notes										0.0		3.1		14.0
Net Income (Loss) Available to Common Stockholders, Diluted										$ 1,056.4		$ 373.6		$ 574.5
Weighted Average Number of Shares Issued, Basic	[1]									299,885,468		294,248,871		262,114,533
Effect of dilutive securities - stock based compensation awards										1,342,247		1,398,329		1,589,286
Effect of dilutive securities - convertible senior notes										0		5,368,876		35,683,554
Weighted Average Number of Shares Outstanding, Diluted		301,557,866	301,384,364	300,752,992	300,172,853	299,080,914	300,374,004	301,075,469	279,528,034	301,227,715	[1]	301,016,076	[1]	299,387,373	[1]
Net income (loss) per share: basic (in dollars per share)		$ 0.92	$ 0.68	$ 0.55	$ 1.38	$ (0.36)	$ 0.92	$ 0.66	$ 0.01	$ 3.52		$ 1.26		$ 2.14
Earnings Per Share, Diluted (in dollars per share)		$ 0.91	$ 0.67	$ 0.55	$ 1.37	$ (0.36)	$ 0.92	$ 0.66	$ 0.01	$ 3.51		$ 1.24		$ 1.92
Stockbased compensation awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total anti-dulitive common share equivalents excluded from earnings per share computations (in shares)										1,200,250		0		1,380,000

[1] In calculating earnings per share, accounting standards require that common shares owned by the Company's Employee Savings and Stock Ownership Plan that are as yet unallocated to participants in the plan be excluded from the calculation. Such shares are issued and outstanding, and have the same voting and other rights applicable to all other common shares.