Borrowings - Narrative (Details)		3 Months Ended
	Jun. 23, 2015 USD ($)	Mar. 31, 2019 USD ($) covenant	Dec. 31, 2018 USD ($)	Jun. 13, 2016 USD ($)
Credit Agreement
Line of Credit Facility [Line Items]
Borrowing capacity	$ 700,000,000
Extension period	1 year
Aggregate lending commitments	$ 350,000,000
Minimum
Line of Credit Facility [Line Items]
Required percent for prepayment amount of aggregate principal amount				5.00%
Minimum | Credit Agreement
Line of Credit Facility [Line Items]
Applicable margin over LIBOR		0.005%
Maximum | Credit Agreement
Line of Credit Facility [Line Items]
Applicable margin over LIBOR		1.50%
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 Facility
Line of Credit Facility [Line Items]
Applicable margin over LIBOR		1.10%
Current borrowings under revolving facility		$ 0
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 | Minimum
Line of Credit Facility [Line Items]
Interest coverage ratio		3.0
Senior Notes | Maximum
Line of Credit Facility [Line Items]
Limitation of outstanding principal of higher preference debt as percent of consolidated assets		15.00%
Leverage ratio		3.50
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 | Credit Agreement
Line of Credit Facility [Line Items]
Borrowing capacity	75,000,000
Swing line Loans | Credit Agreement
Line of Credit Facility [Line Items]
Borrowing capacity	$ 50,000,000
Revolving Facility
Line of Credit Facility [Line Items]
Current borrowings under revolving facility		$ 0	$ 0
Outstanding letters of credit		8,700,000
Amount available to borrow		$ 691,300,000
Number of financial covenants | covenant		2