Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended							9 Months Ended		12 Months Ended
	Dec. 30, 2019	Apr. 30, 2020	Sep. 30, 2018	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	May 31, 2018
Class of Stock [Line Items]
Common Stock, Shares Authorized				90,000,000			90,000,000				90,000,000		90,000,000
Proceeds From Issuance Of Common Stock											$ 629,586	$ 276,960
Common Stock [Member]
Class of Stock [Line Items]
Common Stock, Shares Authorized		2,875,000
Stock Issued During Period, Shares, New Issues		2,875,000		1,515,000	6,851,695	1,400,251		444,251	3,512,500	874,268
Proceeds From Issuance Of Common Stock		$ 170,400
Common Stock [Member] | Forward Sale Agreement [Member]
Class of Stock [Line Items]
Common Stock, Shares Authorized		6,166,666	3,500,000												3,162,500
Stock Issued During Period, Shares, New Issues			3,500,000	1,515,000									3,162,500
Proceeds From Issuance Of Common Stock	$ 195,800		$ 186,000	$ 88,000
At-Market Equity Program [Member] | Common Stock [Member]
Class of Stock [Line Items]
Forward Contract Indexed to Issuer's Equity, Shares						3,169,754
Stock Issued During Period, Shares, New Issues												886,768
Proceeds From Issuance Of Common Stock												$ 58,500
Value Of Equity Instruments To Be Issued				0							0					$ 250,000
At-Market Equity Program Two [Member] | Common Stock [Member]
Class of Stock [Line Items]
Forward Contract Indexed to Issuer's Equity, Shares							2,003,118
Stock Issued During Period, Shares, New Issues								444,228
Proceeds From Issuance Of Common Stock								$ 32,600			359,500
Value Of Equity Instruments To Be Issued														$ 400,000
Common stock Average Price								$ 74.30
At-Market Equity Program Three [Member] | Common Stock [Member]
Class of Stock [Line Items]
Value Of Equity Instruments To Be Issued				$ 0							$ 0
At-Market Equity Program Four [Member]
Class of Stock [Line Items]
Value Of Equity Instruments To Be Issued						$ 400,000
At-Market Equity Program Four [Member] | Common Stock [Member]
Class of Stock [Line Items]
Forward Contract Indexed to Issuer's Equity, Shares											1,832,846
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Shares, at Fair Value				1,628,772							1,628,772
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value				$ 278,000							$ 278,000
Stock Issued During Period, Shares, New Issues											204,074
Proceeds From Issuance Of Common Stock											$ 12,500
Over-Allotment Option [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common Stock, Shares Authorized		375,000
Over-Allotment Option [Member] | Common Stock [Member] | Forward Sale Agreement [Member]
Class of Stock [Line Items]
Common Stock, Shares Authorized															412,500