Borrowings - Information Regarding Outstanding Repurchase Agreement Borrowings During the Period (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Disclosure [Abstract]
Weighted-average outstanding balance	$ 3,841,280	$ 3,819,095	$ 3,730,460	$ 3,956,579
Weighted-average rate	2.17%	1.31%	1.93%	1.10%

The weighted-average carrying value during the period of securities that an institution sells and agrees to repurchase (the identical or substantially the same securities) as a seller-borrower at a specified date for a specified price, also known as a repurchase agreement, or repo. Most repos involve obligations of the federal government or its agencies, but other financial instruments, such as commercial paper, banker's acceptances, and negotiable certificates of deposit, are sometimes used in repos.

No definition available.

Name:	ai_AssetsSoldUnderAgreementsToRepurchaseWeightedAverageOutstandingBalance
Namespace Prefix:	ai_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration