Note Investment Securities Available for Sale (AFS securities in a continuous unrealized loss position) (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 6,197,403	$ 5,641,750
Less than 12 months Gross Unrealized Losses	88,741	90,451
12 months or more Fair Value	357,023	373,987
12 months or more Gross Unrealized Losses	23,845	22,485
Total Fair Value	6,554,426	6,015,737
Total Gross Unrealized Losses	112,586	112,936
US Treasury Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,711,899	1,162,110
Less than 12 months Gross Unrealized Losses	9,520	9,579
12 months or more Fair Value	0	0
12 months or more Gross Unrealized Losses	0	0
Total Fair Value	1,711,899	1,162,110
Total Gross Unrealized Losses	9,520	9,579
Obligations of U.S. Government sponsored entities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	405,934	430,273
Less than 12 months Gross Unrealized Losses	1,797	2,426
12 months or more Fair Value	2,945	3,126
12 months or more Gross Unrealized Losses	34	79
Total Fair Value	408,879	433,399
Total Gross Unrealized Losses	1,831	2,505
Obligations of Puerto Rico, States and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	6,353	6,258
Less than 12 months Gross Unrealized Losses	127	161
12 months or more Fair Value	14,556	16,512
12 months or more Gross Unrealized Losses	8,069	6,092
Total Fair Value	20,909	22,770
Total Gross Unrealized Losses	8,196	6,253
Collateralized Mortgage Obligations - Federal agencies
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	471,245	505,503
Less than 12 months Gross Unrealized Losses	8,778	8,112
12 months or more Fair Value	325,069	339,236
12 months or more Gross Unrealized Losses	15,361	15,933
Total Fair Value	796,314	844,739
Total Gross Unrealized Losses	24,139	24,045
Mortgage Backed Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	3,601,972	3,537,606
Less than 12 months Gross Unrealized Losses	68,519	70,173
12 months or more Fair Value	14,453	15,113
12 months or more Gross Unrealized Losses	381	381
Total Fair Value	3,616,425	3,552,719
Total Gross Unrealized Losses	$ 68,900	$ 70,554