Borrowings - Narrative (Details)				12 Months Ended
	Jun. 23, 2015 USD ($)	Dec. 09, 2011 USD ($)	Dec. 06, 2010 USD ($)	Dec. 31, 2019 USD ($)	May 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 13, 2016 USD ($)
Line of Credit Facility [Line Items]
Credit facility, term	5 years
Balance outstanding				$ 850,622,000		$ 851,078,000
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%
Line of Credit
Line of Credit Facility [Line Items]
Current borrowing capacity	$ 700,000,000
Revolving facility, extension term	1 year
Revolving Credit Facility
Line of Credit Facility [Line Items]
Current borrowing capacity					$ 800,000,000
Aggregate lending commitments					$ 400,000,000
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%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Balance outstanding				$ 0		$ 0
Outstanding letters of credit				8,500,000
Revolving Facility, amount available to borrow				$ 791,500,000
Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Interest coverage ratio				3.0
Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Leverage ratio				3.50
4.20% Senior Notes, due December 2021
Line of Credit Facility [Line Items]
Stated interest rate		4.20%		4.20%		4.20%
Principal amount of private placement		$ 350,000,000.0
Net proceeds from debt offering		346,200,000
Debt offering initial discount		900,000
Debt offering underwriting commission		2,300,000
Estimated offering expenses		600,000
Outstanding repayment of indebtedness		$ 306,000,000.0
Contingent percentage of principal amount plus accrued and unpaid interest		101.00%
4.50% Senior Notes, due December 2020
Line of Credit Facility [Line Items]
Debt instrument face amount			$ 300,000,000.0
Stated interest rate			4.50%	4.50%		4.50%
Net proceeds from debt offering			$ 295,700,000
Debt offering initial discount			1,600,000
Debt offering underwriting commission			1,900,000
Estimated offering expenses			800,000
Outstanding repayment of indebtedness			$ 250,000,000.0
Contingent percentage of principal amount plus accrued and unpaid interest			101.00%
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]
Debt instrument face amount							$ 100,000,000
Stated interest rate				3.20%			3.20%
Balance outstanding				$ 100,000,000		$ 100,000,000
Senior Notes | 3.37% Senior Notes, due June 2025
Line of Credit Facility [Line Items]
Debt instrument face amount							$ 100,000,000
Stated interest rate				3.37%			3.37%
Balance outstanding				$ 100,000,000		100,000,000
Senior Notes | 4.20% Senior Notes, due December 2021
Line of Credit Facility [Line Items]
Stated interest rate				4.20%
Balance outstanding				$ 350,000,000		350,000,000
Senior Notes | 4.50% Senior Notes, due December 2020
Line of Credit Facility [Line Items]
Stated interest rate				4.50%
Balance outstanding				$ 300,000,000		$ 300,000,000
Letters Of Credit | Line of Credit
Line of Credit Facility [Line Items]
Current borrowing capacity	$ 75,000,000
Swing line Loans | Line of Credit
Line of Credit Facility [Line Items]
Current borrowing capacity	$ 50,000,000