Investment Securities - Summary of Continuous Unrealized Loss Position of Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	$ 43,813	$ 1,275,903
Gross Unrealized Holding Losses, Less Than 12 Months	55	49,058
Fair Value, 12 Months or Longer	509,509	89,647
Gross Unrealized Holding Losses, 12 Months or Longer	12,123	5,564
Fair Value	553,322	1,365,550
Gross Unrealized Holding Losses	12,178	54,622
CMO's / REMIC's - residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	0	104,322
Gross Unrealized Holding Losses, Less Than 12 Months	0	1,780
Fair Value, 12 Months or Longer	31,143	17,747
Gross Unrealized Holding Losses, 12 Months or Longer	277	194
Fair Value	31,143	122,069
Gross Unrealized Holding Losses	277	1,974
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	1,953	47,116
Gross Unrealized Holding Losses, Less Than 12 Months	23	3,359
Fair Value, 12 Months or Longer	24,812	10,338
Gross Unrealized Holding Losses, 12 Months or Longer	622	534
Fair Value	26,765	57,454
Gross Unrealized Holding Losses	645	3,893
Government agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	22,224	267,936
Gross Unrealized Holding Losses, Less Than 12 Months	28	20,514
Fair Value, 12 Months or Longer	307,873	38,563
Gross Unrealized Holding Losses, 12 Months or Longer	8,200	3,361
Fair Value	330,097	306,499
Gross Unrealized Holding Losses	8,228	23,875
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	19,636	851,621
Gross Unrealized Holding Losses, Less Than 12 Months	4	23,313
Fair Value, 12 Months or Longer	145,681	22,999
Gross Unrealized Holding Losses, 12 Months or Longer	3,024	1,475
Fair Value	165,317	874,620
Gross Unrealized Holding Losses	3,028	24,788
Other securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	0	4,908
Gross Unrealized Holding Losses, Less Than 12 Months	0	92
Fair Value, 12 Months or Longer	0	0
Gross Unrealized Holding Losses, 12 Months or Longer	0	0
Fair Value	0	4,908
Gross Unrealized Holding Losses	$ 0	$ 92

Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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Amount of unrealized loss on securities classified as available-for-sale.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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-Section 50

-Paragraph 6

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Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

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