Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investment securities available for sale:
Investment securities available for sale	$ 4,801,766	$ 4,967,414
Trading assets	77,112	32,192
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	79,466	33,430
Fair Value, Inputs, Level 1 [Member]
Investment securities available for sale:
Investment securities available for sale	2,610	2,566
Fair Value, Inputs, Level 2 [Member]
Investment securities available for sale:
Investment securities available for sale	4,798,956	4,964,648
Trading assets	79,448	34,907
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	81,802	36,145
Fair Value, Inputs, Level 3 [Member]
Investment securities available for sale:
Investment securities available for sale	200	200
Trading assets	4,270	1,361
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	4,205	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	1,001	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	1,609	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	2,610	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	990,379	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	1,677,246	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	1,410	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	2,126,921	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	4,798,956	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	56,334	29,391
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	59,255	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	3,159	1,532
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	3,073	1,398
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity contract [Member]
Investment securities available for sale:
Trading assets	17,619	1,269
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	17,138	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	2,336	2,715
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,336	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	4,270	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	$ 4,205	$ 0