Borrowings - Narrative (Details)			9 Months Ended
	May 31, 2019 USD ($)	Jun. 23, 2015 USD ($)	Sep. 30, 2019 USD ($) covenant	Dec. 31, 2018	Jun. 13, 2016 USD ($)
Line of Credit Facility [Line Items]
Number of financial covenants | covenant			2
For 12 Months in Connection with Certain Acquisitions
Line of Credit Facility [Line Items]
Leverage ratio			4.00
Minimum
Line of Credit Facility [Line Items]
Required percent for prepayment amount of aggregate principal amount					5.00%
Interest coverage ratio			3.00
Maximum
Line of Credit Facility [Line Items]
Leverage ratio			3.50
Revolving Facility
Line of Credit Facility [Line Items]
Debt instrument, term		5 years
Borrowing capacity	$ 800,000,000	$ 700,000,000
Extension period	1 year
Aggregate lending commitments	$ 400,000,000
Current borrowings under revolving facility			$ 0
Outstanding letters of credit			10,100,000
Amount available to borrow			$ 789,900,000
3.20% Senior Notes, due June 2023
Line of Credit Facility [Line Items]
Stated interest rate			3.20%	3.20%
3.37% Senior Notes, due June 2025
Line of Credit Facility [Line Items]
Stated interest rate			3.37%	3.37%
4.50% Senior Notes, due December 2020
Line of Credit Facility [Line Items]
Stated interest rate			4.50%	4.50%
4.20% Senior Notes, due December 2021
Line of Credit Facility [Line Items]
Stated interest rate			4.20%	4.20%
Senior Notes | Maximum
Line of Credit Facility [Line Items]
Limitation of outstanding principal of higher preference debt as percent of consolidated assets			15.00%
Senior Notes | 3.20% Senior Notes, due June 2023
Line of Credit Facility [Line Items]
Face amount of debt					$ 100,000,000
Stated interest rate					3.20%
Senior Notes | 3.37% Senior Notes, due June 2025
Line of Credit Facility [Line Items]
Face amount of debt					$ 100,000,000
Stated interest rate					3.37%
Letters Of Credit | Revolving Facility
Line of Credit Facility [Line Items]
Borrowing capacity	75,000,000
Swing line Loans | Revolving Facility
Line of Credit Facility [Line Items]
Borrowing capacity	$ 50,000,000