Investment Securities - Cost basis, unrealized gains and losses, and fair value of securities available for sale (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Investment securities available-for-sale:
Amortized Cost	$ 1,890,440		$ 1,817,774		$ 1,879,247
Gross Unrealized Gains	26,995		26,852		34,819
Gross Unrealized Losses	8,009		10,033		15,251
Fair value	1,909,426		1,834,593		1,898,815
U.S. Government agencies
Investment securities available-for-sale:
Amortized Cost					43,593
Gross Unrealized Gains					286
Gross Unrealized Losses					90
Fair value					43,789
State and political subdivisions
Investment securities available-for-sale:
Amortized Cost	22,711		27,717		21,490
Gross Unrealized Gains	1,330		1,354		1,321
Gross Unrealized Losses	4		19		3
Fair value	24,037		29,052		22,808
Mortgage-backed securities
Investment securities available-for-sale:
Amortized Cost	1,450,645	[1]	1,408,042	[1]	1,692,446	[1]
Gross Unrealized Gains	21,210	[1]	22,552	[1]	33,212	[1]
Gross Unrealized Losses	3,544	[1]	2,092	[1]	590	[1]
Fair value	1,468,311	[1]	1,428,502	[1]	1,725,068	[1]
Corporate bonds
Investment securities available-for-sale:
Amortized Cost	190,843		169,783		119,154
Gross Unrealized Gains	2,035		1,052
Gross Unrealized Losses	4,084		7,173		14,568
Fair value	188,794		163,662		104,586
Asset-backed securities
Investment securities available-for-sale:
Amortized Cost	223,827		209,411
Gross Unrealized Gains	2,420		1,894
Gross Unrealized Losses	377		749
Fair value	225,870		210,556
Other
Investment securities available-for-sale:
Amortized Cost	2,414		2,821		2,564
Gross Unrealized Gains
Gross Unrealized Losses
Fair value	$ 2,414		$ 2,821		$ 2,564

[1] All are residential type mortgage-backed securities