Fair Value Measurements - Carrying value and estimated fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets:
Cash and cash equivalents	$ 3,523,332	$ 4,082,826	$ 3,959,932
Debt securities held to maturity	1,975,729	1,046,093
Investment securities held to maturity, estimated fair value	1,967,996	1,040,543
Liabilities:
Deposits	69,940,412	69,256,313
Federal funds purchased and securities sold under agreements to repurchase	5,933	19,591
Fair Value, Inputs, Level 1
Assets:
Cash and cash equivalents, estimated fair value	3,523,332	4,082,826
Fair Value, Inputs, Level 2
Assets:
Investment securities held to maturity, estimated fair value	934,348
Liabilities:
Deposits, estimated fair value	69,990,606	69,302,597
FHLB and other borrowings, estimated fair value	3,355,549	4,010,308
Federal funds purchased and securities sold under agreements to repurchase, estimated fair value	5,933	19,591
Fair Value, Inputs, Level 3
Assets:
Investment securities held to maturity, estimated fair value	1,033,648	1,040,543
Loans, net, estimated fair value	58,822,080	57,906,982
Reported value measurement
Assets:
Cash and cash equivalents	3,523,332	4,082,826
Debt securities held to maturity	1,975,729	1,046,093
Loans, net	61,375,790	60,781,008
Liabilities:
Deposits	69,940,412	69,256,313
FHLB and other borrowings	3,322,940	3,959,930
Federal funds purchased and securities sold under agreements to repurchase	5,933	19,591
Estimate of fair value measurement
Assets:
Cash and cash equivalents, estimated fair value	3,523,332	4,082,826
Investment securities held to maturity, estimated fair value	1,967,996	1,040,543
Loans, net, estimated fair value	58,822,080	57,906,982
Liabilities:
Deposits, estimated fair value	69,990,606	69,302,597
FHLB and other borrowings, estimated fair value	3,355,549	4,010,308
Federal funds purchased and securities sold under agreements to repurchase, estimated fair value	$ 5,933	$ 19,591