Borrowings - Senior Notes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Sep. 30, 2018	Mar. 31, 2014	Mar. 31, 2013	Jan. 31, 2013
Debt Instrument [Line Items]
Interest Expense	$ 19,700,000	$ 16,200,000	$ 59,200,000	$ 47,700,000
3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Interest rate on senior notes	3.875%		3.875%
5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Interest rate on senior notes	5.625%		5.625%
5.650% Senior Notes Due 9/15/2048
Debt Instrument [Line Items]
Interest rate on senior notes	5.65%		5.65%
3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Interest rate on senior notes	3.50%		3.50%
Senior Notes | 3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Interest rate on senior notes	3.875%		3.875%			3.875%
Aggregate Principal Amount	$ 250,000,000.0		$ 250,000,000.0
Fair Value	268,900,000		268,900,000		$ 262,800,000
Interest Expense	2,500,000	2,500,000	7,500,000	7,300,000
Senior notes percentage of par value									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				$ 200,000,000.0	$ 400,000,000.0
Fair Value	750,300,000		750,300,000		713,400,000
Interest Expense	8,400,000	8,300,000	25,300,000	25,100,000
Senior notes percentage of par value							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	454,200,000		454,200,000		424,000,000.0
Interest Expense	$ 5,000,000.0	4,900,000	$ 14,900,000	14,800,000
Senior notes percentage of par value						99.914%
Senior Notes | 3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Interest rate on senior notes	3.50%		3.50%
Aggregate Principal Amount	$ 425,000,000.0		$ 425,000,000.0
Fair Value	463,300,000		463,300,000		$ 430,200,000
Interest Expense	$ 3,800,000	$ 500,000	$ 11,500,000	$ 500,000
Senior notes percentage of par value		99.841%		99.841%
Senior Notes | 3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Repurchased principal amount						$ 250,000,000.0