Assets and Liabilities Measured at Fair Value - Assets and liabilities measured at fair value on recurring basis (Details) (Recurring, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Level 1
Assets:
Assets, fair value	$ 3,037	$ 3,101	$ 2,973
Liabilities:
Liabilities, fair value	3,037	3,101	2,973
Level 1 | Deferred compensation plan liability
Liabilities:
Liabilities, fair value	3,037	3,101	2,973
Level 1 | Brokered certificates of deposit
Liabilities:
Liabilities, fair value
Level 1 | Derivative financial instruments
Assets:
Assets, fair value
Liabilities:
Liabilities, fair value
Level 1 | U.S. Government agencies
Assets:
Assets, fair value
Level 1 | State and political subdivisions
Assets:
Assets, fair value
Level 1 | Mortgage-backed securities
Assets:
Assets, fair value
Level 1 | Corporate bonds
Assets:
Assets, fair value
Level 1 | Asset-backed securities
Assets:
Assets, fair value
Level 1 | Other
Assets:
Assets, fair value
Level 1 | Deferred compensation plan assets
Assets:
Assets, fair value	3,037	3,101	2,973
Level 2
Assets:
Assets, fair value	1,909,678	1,834,901	1,898,538
Liabilities:
Liabilities, fair value	208,971	167,184	63,668
Level 2 | Deferred compensation plan liability
Liabilities:
Liabilities, fair value
Level 2 | Brokered certificates of deposit
Liabilities:
Liabilities, fair value	194,415	154,641	61,069
Level 2 | Derivative financial instruments
Assets:
Assets, fair value	602	658	73
Liabilities:
Liabilities, fair value	14,556	12,543	2,599
Level 2 | U.S. Government agencies
Assets:
Assets, fair value			43,789
Level 2 | State and political subdivisions
Assets:
Assets, fair value	24,037	29,052	22,808
Level 2 | Mortgage-backed securities
Assets:
Assets, fair value	1,468,311	1,428,502	1,725,068
Level 2 | Corporate bonds
Assets:
Assets, fair value	188,444	163,312	104,236
Level 2 | Asset-backed securities
Assets:
Assets, fair value	225,870	210,556
Level 2 | Other
Assets:
Assets, fair value	2,414	2,821	2,564
Level 2 | Deferred compensation plan assets
Assets:
Assets, fair value
Level 3
Assets:
Assets, fair value	350	350	350
Liabilities:
Liabilities, fair value
Level 3 | Deferred compensation plan liability
Liabilities:
Liabilities, fair value
Level 3 | Brokered certificates of deposit
Liabilities:
Liabilities, fair value
Level 3 | Derivative financial instruments
Assets:
Assets, fair value
Liabilities:
Liabilities, fair value
Level 3 | U.S. Government agencies
Assets:
Assets, fair value
Level 3 | State and political subdivisions
Assets:
Assets, fair value
Level 3 | Mortgage-backed securities
Assets:
Assets, fair value
Level 3 | Corporate bonds
Assets:
Assets, fair value	350	350	350
Level 3 | Asset-backed securities
Assets:
Assets, fair value
Level 3 | Other
Assets:
Assets, fair value
Level 3 | Deferred compensation plan assets
Assets:
Assets, fair value
Total
Assets:
Assets, fair value	1,913,065	1,838,352	1,901,861
Liabilities:
Liabilities, fair value	212,008	170,285	66,641
Total | Deferred compensation plan liability
Liabilities:
Liabilities, fair value	3,037	3,101	2,973
Total | Brokered certificates of deposit
Liabilities:
Liabilities, fair value	194,415	154,641	61,069
Total | Derivative financial instruments
Assets:
Assets, fair value	602	658	73
Liabilities:
Liabilities, fair value	14,556	12,543	2,599
Total | U.S. Government agencies
Assets:
Assets, fair value			43,789
Total | State and political subdivisions
Assets:
Assets, fair value	24,037	29,052	22,808
Total | Mortgage-backed securities
Assets:
Assets, fair value	1,468,311	1,428,502	1,725,068
Total | Corporate bonds
Assets:
Assets, fair value	188,794	163,662	104,586
Total | Asset-backed securities
Assets:
Assets, fair value	225,870	210,556
Total | Other
Assets:
Assets, fair value	2,414	2,821	2,564
Total | Deferred compensation plan assets
Assets:
Assets, fair value	$ 3,037	$ 3,101	$ 2,973