Summary of Significant Accounting Policies - Merchandise Inventories (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Percentage of inventories, cost determined using LIFO	70.00%				75.00%				70.00%	75.00%
Excess cost of inventories over LIFO, if used FIFO	$ 1,519,200				$ 1,511,800				$ 1,519,200	$ 1,511,800
LIFO charges (credit)	$ (35,800)	$ 6,100	$ 23,900	$ 13,300	$ 57,200	$ (9,900)	$ (66,800)	$ (3,000)	$ 7,422	$ (22,544)	$ 67,324