Fair Value Measurements - Summary of Carrying Values, Estimated Fair Values and Fair Value Measurement Levels of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Financial Assets:
Securities held to maturity	$ 7,317,015	[1]	$ 7,445,244	[2]
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents	662,537		644,550
Financial Liabilities:
Deposits	21,983,386	[3]	18,728,896	[3]
Significant Other Observable Inputs (Level 2)
Financial Assets:
Securities held to maturity	7,315,978		7,438,091
FHLB stock	520,445	[4]	561,390	[4]
Financial Liabilities:
Deposits	6,346,755	[5]	6,983,492	[5]
Borrowed funds	15,256,216		16,058,931
Significant Unobservable Inputs (Level 3)
Financial Assets:
Securities held to maturity	1,037		7,153
Loans, net	35,837,523		32,628,361
Carrying Value
Financial Assets:
Cash and cash equivalents	662,537		644,550
Securities held to maturity	7,268,244		7,670,282
FHLB stock	520,445	[4]	561,390	[4]
Loans, net	35,251,352		32,727,507
Financial Liabilities:
Deposits	28,307,771		25,660,992
Borrowed funds	14,396,912		15,105,002
Estimated Fair Value
Financial Assets:
Cash and cash equivalents	662,537		644,550
Securities held to maturity	7,317,015		7,445,244
FHLB stock	520,445	[4]	561,390	[4]
Loans, net	35,837,523		32,628,361
Financial Liabilities:
Deposits	28,330,141		25,712,388
Borrowed funds	$ 15,256,216		$ 16,058,931

[1] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At September 30, 2014, the non-credit portion of OTTI recorded in AOCL was $8.9 million (before taxes).

[2] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At December 31, 2013, the non-credit portion of OTTI recorded in AOCL was $9.2 million (before taxes).

[3] NOW and money market accounts, savings accounts, and non-interest-bearing accounts.

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

[5] Certificates of deposit.