Investment in Qualified Affordable Housing (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Noninterest Expense	$ 47,429	$ 44,972	$ 137,816	$ 136,992
Income Tax Expense (Benefit)	8,033	8,363	24,433	25,801
Affordable Housing Program Assessments	1,700	1,700	5,200
Net Income	20,040	18,269	60,123	59,656
Other Assets	11,114		11,114		$ 14,645
Assets	7,300,340	7,011,255	7,300,340	7,011,255	7,001,199
Retained earnings	501,145		501,145		484,484
Affordable Housing Program Obligation	21,339		21,339		16,629
Affordable Housing Tax Credits and Other Tax Benefits, Amount	2,300	2,200	6,800	6,600
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	715,803		715,803		696,541
Other Assets [Member] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets					140,803
Other Assets [Member] | Restatement Adjustment [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets					138,746
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income			60,123	59,656
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	501,145	474,913	501,145	474,913	486,541	$ 460,643
Retained Earnings [Member] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings					486,541
Retained Earnings [Member] | Restatement Adjustment [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings					484,484
Assets [Member] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets					7,003,256
Assets [Member] | Restatement Adjustment [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets					7,001,199
Equity [Member] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest					698,598
Equity [Member] | Restatement Adjustment [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest					696,541
Qualified Affordable Housing [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity Method Investments	$ 52,731		$ 52,731		48,911
Qualified Affordable Housing [Domain] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity Method Investments					45,400
Other Income [Member] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Noninterest Expense		46,903		142,797	195,234
Other Income [Member] | Restatement Adjustment [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Noninterest Expense		44,972		136,992	187,510
Income Tax Expense [Member] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income Tax Expense (Benefit)		6,398		19,903	28,602
Income Tax Expense [Member] | Restatement Adjustment [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income Tax Expense (Benefit)		8,363		25,801	36,459
Net Income [Domain] | Scenario, Previously Reported [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income		18,303		59,749	84,090
Net Income [Domain] | Restatement Adjustment [Member] | Investment, Name [Domain]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income		$ 18,269		$ 59,656	$ 83,957