Fair Value Information - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	$ 10,080	$ 10,846
Interest rate swaps	573,707	654,097
Interest rate swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	0	0
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	10,080	10,846
Interest rate swaps	10,080	10,846
Fair Value on Recurring Basis [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Total liabilities	0	0
Fair Value on Recurring Basis [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	3,147,238	2,674,488
Total liabilities	10,080	10,846
Fair Value on Recurring Basis [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Total liabilities	0	0
Fair Value on Recurring Basis [Member] | Government agency [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Government agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	330,843	326,525
Fair Value on Recurring Basis [Member] | Government agency [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Residential mortgage-backed securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS		0
Fair Value on Recurring Basis [Member] | Residential mortgage-backed securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS		1,379,943
Fair Value on Recurring Basis [Member] | Residential mortgage-backed securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS		0
Fair Value on Recurring Basis [Member] | CMO's / REMIC's - residential [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | CMO's / REMIC's - residential [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	304,091	366,175
Fair Value on Recurring Basis [Member] | CMO's / REMIC's - residential [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Municipal bonds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Municipal bonds [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	579,641	586,091
Fair Value on Recurring Basis [Member] | Municipal bonds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Other securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Other securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	5,087	4,908
Fair Value on Recurring Basis [Member] | Other securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Investment securities - AFS [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Investment securities - AFS [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	3,137,158	2,663,642
Fair Value on Recurring Basis [Member] | Investment securities - AFS [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0	0
Fair Value on Recurring Basis [Member] | Interest rate swaps [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	0	0
Interest rate swaps	0	0
Fair Value on Recurring Basis [Member] | Interest rate swaps [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	10,080	10,846
Interest rate swaps	10,080	10,846
Fair Value on Recurring Basis [Member] | Interest rate swaps [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	0	0
Interest rate swaps	0	0
Fair Value on Recurring Basis [Member] | Residential Mortgage Backed Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0
Fair Value on Recurring Basis [Member] | Residential Mortgage Backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	1,917,496
Fair Value on Recurring Basis [Member] | Residential Mortgage Backed Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	0
Fair Value on Recurring Basis [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	3,147,238	2,674,488
Total liabilities	10,080	10,846
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | Government agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	330,843	326,525
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS		1,379,943
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | CMO's / REMIC's - residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	304,091	366,175
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | Municipal bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	579,641	586,091
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | Other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	5,087	4,908
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | Investment securities - AFS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	3,137,158	2,663,642
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | Interest rate swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	10,080	10,846
Interest rate swaps	10,080	10,846
Fair Value on Recurring Basis [Member] | Carrying Value [Member] | Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - AFS	$ 1,917,496