Investment securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Debt securities available for sale:
Due in one year or less	$ 24,442
Due after one year through five years	22,459
Due after five years through ten years	9,221
Due after ten years	181,932
Total available for sale (amortized cost)	238,054
Mortgage-backed securities available for sale	3,940,009
Total	4,178,063
Debt securities held to maturity:
Due in one year or less	25,558
Due after one year through five years	55,547
Due after five years through ten years	99,177
Due after ten years	10,211
Total available for held to maturity (amortized cost)	190,493
Mortgage-backed securities held to maturity	768,706
Total	959,199
Debt securities available for sale:
Due in one year or less	24,602
Due after one year through five years	23,522
Due after five years through ten years	9,843
Due after ten years	181,003
Total available for available for sale (fair value)	238,970
Mortgage-backed securities available for sale	4,023,157
Total	4,262,127
Debt securities held to maturity:
Due in one year or less	25,691
Due after one year through five years	57,974
Due after five years through ten years	103,589
Due after ten years	10,211
Total available for held to maturity (fair value)	197,465
Mortgage-backed securities held to maturity	713,318
Total	$ 910,783