Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investment securities available for sale:
Investment securities available for sale	$ 4,905,841	$ 4,967,414
Trading assets	53,087	32,192
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	55,223	33,430
Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale:
Investment securities available for sale	2,605	2,566
Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale:
Investment securities available for sale	4,903,036	4,964,648
Trading assets	55,315	34,907
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	57,451	36,145
Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale:
Investment securities available for sale	200	200
Trading assets	2,689	1,361
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,732	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale	1,000	997
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Investment securities available for sale	1,605	1,569
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Available-for-sale Securities [Member]
Investment securities available for sale:
Investment securities available for sale	2,605	2,566
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Government-Sponsored Enterprise (GSE) [Member]
Investment securities available for sale:
Investment securities available for sale	1,303,404	1,414,626
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | GNMA [Member]
Investment securities available for sale:
Investment securities available for sale	1,547,255	1,590,003
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Private Label [Member]
Investment securities available for sale:
Investment securities available for sale	1,512	1,709
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | GNMA Commercial Mortgage-Related Securities [Member]
Investment securities available for sale:
Investment securities available for sale	2,047,865	1,955,310
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Investment securities available for sale	3,000	3,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Available-for-sale Securities [Member]
Investment securities available for sale:
Investment securities available for sale	4,903,036	4,964,648
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate-related instruments - customer and mirror [Member]
Investment securities available for sale:
Trading assets	47,780	29,391
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	50,145	30,886
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign currency exchange forwards [Member]
Investment securities available for sale:
Trading assets	4,203	1,532
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	4,097	1,398
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity contract [Member]
Investment securities available for sale:
Trading assets	1,104	1,269
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	981	1,146
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Written options (time deposit) [Member]
Investment securities available for sale:
Trading assets	2,228	2,715
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,228	2,715
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Trading assets	200	200
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale Securities [Member]
Investment securities available for sale:
Investment securities available for sale	200	200
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Interest rate lock commitments (mortgage) [Member]
Investment securities available for sale:
Trading assets	2,689	958
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Forward commitments (mortgage) [Member]
Investment securities available for sale:
Trading assets	0	403
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	$ 1,732	$ 0