Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
FINANCIAL ASSETS
Cash and cash equivalents	$ 1,856,437	$ 1,857,535	$ 1,720,239	$ 1,945,148
Securities held for investment	9,297
Loans, net	3,622,611	3,348,112	3,206,102
FINANCIAL LIABILITIES
Deposits	5,721,593	5,419,519	5,150,411
Short-term borrowings	12,317	4,590	3,522
Long-term borrowings		6,938	9,964
Junior subordinated debentures	26,804	26,804	26,804
Carrying Amount [Member] | Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents	1,856,437		1,720,239
Securities held for investment	9,297		14,511
FINANCIAL LIABILITIES
Deposits	5,721,593		5,150,411
Short-term borrowings	12,317		3,522
Long-term borrowings			9,964
Junior subordinated debentures	26,804		26,804
Carrying Amount [Member] | Level 3 Inputs [Member]
FINANCIAL ASSETS
Loans, net	3,622,611		3,206,102
Fair Value [Member]
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments	1,791		1,574
Letters of credit	426		437
Fair Value [Member] | Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents	1,856,437		1,720,239
Securities held for investment	9,386		14,654
FINANCIAL LIABILITIES
Deposits	5,754,876		5,175,528
Short-term borrowings	12,317		3,522
Long-term borrowings			9,906
Junior subordinated debentures	28,948		28,991
Fair Value [Member] | Level 3 Inputs [Member]
FINANCIAL ASSETS
Loans, net	$ 3,661,752		$ 3,238,652