Borrowings - Narrative (Details)		6 Months Ended
	May 31, 2019 USD ($)	Jun. 30, 2019 USD ($) covenant	Dec. 31, 2018	Jun. 13, 2016 USD ($)	Jun. 23, 2015 USD ($)
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]
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		9,100,000
Amount available to borrow		$ 790,900,000
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