FHLB and Other Borrowings - Debt (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
FHLB and other borrowings		$ 5,438,620,000	$ 4,809,843,000
FHLB Advances
Debt Instrument [Line Items]
FHLB and other borrowings		2,943,044,000	2,988,441,000
Unamortized (discount) premium		$ (27,000)	(102,000)
FHLB Advances | LIBOR-based floating rate FHLB advances
Debt Instrument [Line Items]
Maturity date, start		Jan. 01, 2016
Maturity date, end		Dec. 31, 2021
FHLB and other borrowings		$ 525,000,000	$ 525,000,000
Stated interest rate		0.62%	0.62%
FHLB Advances | Fixed rate, FHLB advances
Debt Instrument [Line Items]
Maturity date, start		Jan. 01, 2015
Maturity date, end		Dec. 31, 2035
FHLB and other borrowings		$ 2,418,071,000	$ 2,463,543,000
Stated interest rate		1.08%	1.08%
Senior notes | 1.85% senior notes
Debt Instrument [Line Items]
Maturity date		Sep. 29, 2017	Sep. 29, 2017
FHLB and other borrowings		$ 400,000,000	$ 400,000,000
Stated interest rate		1.85%	1.85%
Senior notes | 2.75% senior notes
Debt Instrument [Line Items]
Maturity date		Sep. 29, 2019	Sep. 29, 2019
FHLB and other borrowings		$ 600,000,000	$ 600,000,000
Stated interest rate		2.75%	2.75%
Subordinated debentures
Debt Instrument [Line Items]
FHLB and other borrowings		$ 2,391,875,000	$ 1,717,668,000
Unamortized (discount) premium		$ (22,359,000)	(15,135,000)
Subordinated debentures | 6.40% subordinated debentures
Debt Instrument [Line Items]
Maturity date		Oct. 01, 2017
FHLB and other borrowings		$ 350,000,000	$ 350,000,000
Stated interest rate		6.40%	6.40%
Subordinated debentures | 5.50% subordinated debentures
Debt Instrument [Line Items]
Maturity date		Apr. 01, 2020
FHLB and other borrowings		$ 227,764,000	$ 227,764,000
Stated interest rate		5.50%	5.50%
Subordinated debentures | Subordinated Debt, 3.88% Interest Rate
Debt Instrument [Line Items]
Maturity date	Apr. 10, 2025	Apr. 10, 2025
FHLB and other borrowings	$ 700,000,000	$ 700,000,000	$ 0
Stated interest rate		3.875%	3.875%
Subordinated debentures | 5.90% subordinated debentures
Debt Instrument [Line Items]
Maturity date		Apr. 01, 2026
FHLB and other borrowings		$ 71,086,000	$ 71,086,000
Stated interest rate		5.90%	5.90%
Subordinated debentures | Fair value of hedged subordinated debentures
Debt Instrument [Line Items]
FHLB and other borrowings		$ 65,384,000	$ 83,953,000
Capital Securities
Debt Instrument [Line Items]
FHLB and other borrowings		103,701,000	103,734,000
Unamortized (discount) premium		$ 600,000	633,000
Capital Securities | LIBOR plus 3.05% floating rate debentures payable to State National Capital Trust I
Debt Instrument [Line Items]
Maturity date		Sep. 30, 2033
FHLB and other borrowings		$ 15,470,000	$ 15,470,000
Stated interest rate		3.05%	3.05%
Capital Securities | LIBOR plus 2.85% floating rate debentures payable to Texas Regional Statutory Trust I
Debt Instrument [Line Items]
Maturity date		Mar. 17, 2034
FHLB and other borrowings		$ 51,547,000	$ 51,547,000
Stated interest rate		2.85%	2.85%
Capital Securities | LIBOR plus 2.60% floating rate debentures payable to TexasBanc Capital Trust I
Debt Instrument [Line Items]
Maturity date		Jul. 23, 2034
FHLB and other borrowings		$ 25,774,000	$ 25,774,000
Stated interest rate		2.60%	2.60%
Capital Securities | LIBOR plus 2.79% floating rate debentures payable to State National Statutory Trust II
Debt Instrument [Line Items]
Maturity date		Mar. 17, 2034
FHLB and other borrowings		$ 10,310,000	$ 10,310,000
Stated interest rate		2.79%	2.79%