Financing Agreement - Summary of Activity Under Credit Arrangements, Excluding Acquired Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Total borrowings at December 31	$ 1,365,000	$ 1,600,100	$ 1,862,600
Average outstanding borrowings during year	1,879,700	1,912,000	1,474,500
Maximum outstanding borrowings during year	$ 2,370,600	$ 2,127,900	$ 1,999,700
Weighted-average interest rate at December 31	2.30%	3.29%	3.64%