Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Financial Assets:
Cash and cash equivalents, Carrying Value		$ 1,458,641	$ 741,870	$ 1,474,955
Cash and cash equivalents, Estimated Fair Value		1,458,641	741,870
FHLB stock, Carrying Value	[1]	697,483	647,562
FHLB stock, Estimated Fair Value	[1]	697,483	647,562
Loans, net, Carrying Value		42,758,184	41,746,517
Loans, net, Estimated Fair Value		42,351,344	41,699,929
Financial Liabilities:
Deposits, Carrying Value		31,704,941	31,657,132
Deposits, Estimated Fair Value		31,748,169	31,713,945
Borrowed funds, Carrying Value		15,683,303	14,557,593
Borrowed funds, Estimated Fair Value		16,447,187	14,882,776
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents, Estimated Fair Value		1,458,641	741,870
Financial Liabilities:
Deposits, Estimated Fair Value	[2]	20,714,861	17,442,274
Significant Other Observable Inputs (Level 2)
Financial Assets:
FHLB stock, Estimated Fair Value	[1]	697,483	647,562
Financial Liabilities:
Deposits, Estimated Fair Value	[3]	11,033,308	14,271,671
Borrowed funds, Estimated Fair Value		16,447,187	14,882,776
Significant Unobservable Inputs (Level 3)
Financial Assets:
Loans, net, Estimated Fair Value		$ 42,351,344	$ 41,699,929

[1] Carrying value and estimated fair value are at cost.

[2] Interest-bearing checking and money market accounts, savings accounts, and non-interest-bearing accounts.

[3] Certificates of deposit.