Borrowings - Senior Notes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2014	Mar. 31, 2013	Jan. 31, 2013
Debt Instrument [Line Items]
Interest Expense	$ 19,700,000	$ 19,700,000	$ 59,200,000	$ 59,200,000
3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Interest rate (percent)	3.875%		3.875%
5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Interest rate (percent)	5.625%		5.625%
5.650% Senior Notes Due 9/15/2048
Debt Instrument [Line Items]
Interest rate (percent)	5.65%		5.65%
3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Interest rate (percent)	3.50%		3.50%
Senior Notes | 3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Interest rate (percent)	3.875%		3.875%
Aggregate Principal Amount	$ 250,000,000.0		$ 250,000,000.0
Fair Value	262,100,000		262,100,000		$ 270,000,000.0
Interest Expense	2,500,000	2,500,000	7,500,000	7,500,000
Senior notes percentage of par value (percent)										99.966%
Senior Notes | 5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Aggregate Principal Amount	600,000,000.0		600,000,000.0					$ 200,000,000	$ 400,000,000
Fair Value	790,000,000.0		790,000,000.0		782,600,000
Interest Expense	8,400,000	8,400,000	25,300,000	25,300,000
Senior notes percentage of par value (percent)								104.315%	99.583%
Senior Notes | 5.650% Senior Notes Due 9/15/2048
Debt Instrument [Line Items]
Aggregate Principal Amount	350,000,000.0		350,000,000.0
Fair Value	482,700,000		482,700,000		469,300,000
Interest Expense	$ 5,000,000.0	5,000,000.0	$ 14,900,000	14,900,000
Senior notes percentage of par value (percent)							99.914%
Senior Notes | 3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Interest rate (percent)	3.50%		3.50%
Aggregate Principal Amount	$ 425,000,000.0		$ 425,000,000.0
Fair Value	456,800,000		456,800,000		$ 476,600,000
Interest Expense	$ 3,800,000	$ 3,800,000	$ 11,500,000	$ 11,500,000
Senior notes percentage of par value (percent)						99.841%