Financing Agreement - Summary of Activity Under Credit Arrangements, Excluding Acquired Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Total borrowings at December 31	$ 883,000	$ 1,112,142	$ 425,707
Average outstanding borrowings during year	1,133,641	638,614	403,097
Maximum outstanding borrowings during year	$ 1,319,105	$ 1,234,337	$ 523,097
Interest rate at December 31	2.91%	2.25%	2.33%