Borrowings - Narrative (Details)				12 Months Ended
	Jun. 23, 2015 USD ($)	Dec. 09, 2011 USD ($)	Dec. 06, 2010 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 13, 2016 USD ($)
Line of Credit Facility [Line Items]
Credit facility, term	5 years
Revolving facility expiration date				Jun. 23, 2020
Current borrowings under revolving facility					$ 10,740,000
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
Aggregate lending commitments	$ 350,000,000
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]
Applicable margin over LIBOR				1.10%
Current borrowings under revolving facility				$ 0
Outstanding letters of credit				8,800,000
Revolving Facility, amount available to borrow				$ 691,200,000
Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Applicable margin over LIBOR				0.005%
Interest coverage ratio				3.0
Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Applicable margin over LIBOR				1.50%
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
Debt instrument maturity period		Dec. 15, 2021		Dec. 31, 2021	Dec. 31, 2021
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
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%
Debt instrument maturity period			Dec. 15, 2020	Dec. 31, 2020	Dec. 31, 2020
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
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%
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%
Senior Notes | 4.20% Senior Notes, due December 2021
Line of Credit Facility [Line Items]
Stated interest rate				4.20%
Senior Notes | 4.50% Senior Notes, due December 2020
Line of Credit Facility [Line Items]
Stated interest rate				4.50%
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