Note Borrowings (Repurchase agreements accounted for as secured borrowings) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 200,871	$ 281,529
Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	200,871	281,529
US Treasury Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	170,077	237,621
US Treasury Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	31,494	138,689
US Treasury Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	14,687	79,374
US Treasury Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	123,896	19,558
Obligations of U.S. Government sponsored entities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	0	6,055
Obligations of U.S. Government sponsored entities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	0	6,055
Mortgage Backed Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	27,146	27,324
Mortgage Backed Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	27,146	6,859
Mortgage Backed Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	0	20,465
CMO | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	3,648	10,529
CMO | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 3,648	$ 10,529