Securities Financing Activities - Contractual Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	$ 4,328,851	$ 650,618
Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	1,610,392	321,310
Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	1,870,709	0
30 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	211,000	23,558
Greater Than 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	636,750	305,750
U.S. Treasury and other U.S. government agencies
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	4,328,851	627,060
U.S. Treasury and other U.S. government agencies | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	1,610,392	321,310
U.S. Treasury and other U.S. government agencies | Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	1,870,709	0
U.S. Treasury and other U.S. government agencies | 30 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	211,000	0
U.S. Treasury and other U.S. government agencies | Greater Than 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	636,750	305,750
Mortgage-backed securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	0	23,558
Mortgage-backed securities | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	0	0
Mortgage-backed securities | Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	0	0
Mortgage-backed securities | 30 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	0	23,558
Mortgage-backed securities | Greater Than 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements repurchase	$ 0	$ 0