Note Investment Securities Available for Sale (AFS securities in a continuous unrealized loss position) (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 6,099,123	$ 5,641,750
Less than 12 months Gross Unrealized Losses	61,694	90,451
12 months or more Fair Value	763,765	373,987
12 months or more Gross Unrealized Losses	22,822	22,485
Total Fair Value	6,862,888	6,015,737
Total Gross Unrealized Losses	84,516	112,936
US Treasury Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	2,195,874	1,162,110
Less than 12 months Gross Unrealized Losses	10,309	9,579
12 months or more Fair Value	0	0
12 months or more Gross Unrealized Losses	0	0
Total Fair Value	2,195,874	1,162,110
Total Gross Unrealized Losses	10,309	9,579
Obligations of U.S. Government sponsored entities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	496,345	430,273
Less than 12 months Gross Unrealized Losses	1,797	2,426
12 months or more Fair Value	24,139	3,126
12 months or more Gross Unrealized Losses	97	79
Total Fair Value	520,484	433,399
Total Gross Unrealized Losses	1,894	2,505
Obligations of Puerto Rico, States and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	0	6,258
Less than 12 months Gross Unrealized Losses	0	161
12 months or more Fair Value	0	16,512
12 months or more Gross Unrealized Losses	0	6,092
Total Fair Value	0	22,770
Total Gross Unrealized Losses	0	6,253
Collateralized Mortgage Obligations - Federal agencies
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	364,098	505,503
Less than 12 months Gross Unrealized Losses	5,401	8,112
12 months or more Fair Value	387,284	339,236
12 months or more Gross Unrealized Losses	14,945	15,933
Total Fair Value	751,382	844,739
Total Gross Unrealized Losses	20,346	24,045
Mortgage Backed Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	3,042,806	3,537,606
Less than 12 months Gross Unrealized Losses	44,187	70,173
12 months or more Fair Value	352,342	15,113
12 months or more Gross Unrealized Losses	7,780	381
Total Fair Value	3,395,148	3,552,719
Total Gross Unrealized Losses	$ 51,967	$ 70,554