Notes Payable - Note Issuances (Details) - USD ($) $ in Thousands	1 Months Ended			6 Months Ended
	Apr. 30, 2018	Mar. 31, 2017	Sep. 30, 2014	Jun. 30, 2018	Jun. 30, 2017
Debt
Proceeds from issuance of debt				$ 497,500	$ 711,812
Notes Payable
Debt
Issuance of Debt	$ 500,000
Effective yield (as a percent)	3.875%
Public offering price	99.50%
Notes Payable | 3.957% notes, issued in April 2018 and due in April 2045
Debt
Effective yield (as a percent)	3.957%
Notes Payable | Notes issued in April 2018
Debt
Proceeds from issuance of debt	$ 493,100
Notes Payable | 4.650% notes, issued in March 2017 and due in March 2047
Debt
Issuance of Debt		$ 300,000
Effective yield (as a percent)		4.65%
Public offering price		99.97%
Notes Payable | 4.125% notes, issued in March 2017 and due in October 2026
Debt
Stated interest rate (as a percent)		4.125%
Issuance of Debt		$ 400,000
Effective yield (as a percent)		3.75%
Public offering price		102.98%
Notes Payable | 4.125% notes, issued in September 2014 and due in October 2026
Debt
Issuance of Debt			$ 250,000
Notes Payable | Notes issued in March 2017
Debt
Proceeds from issuance of debt		$ 705,200