Financing Agreement - Summary of Activity Under Credit Arrangements, Excluding Acquired Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Total borrowings at December 31	$ 1,862,631	$ 883,000	$ 1,112,142
Average outstanding borrowings during year	1,474,485	1,133,641	638,614
Maximum outstanding borrowings during year	$ 1,999,731	$ 1,319,105	$ 1,234,337
Interest rate at December 31	3.64%	2.91%	2.25%