Loans and Allowance for Losses and Concentrations of Credit Risk - Certain Loans Acquired Delinquencies and Net Credit Losses (Details) (USD $)	9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 86,487,000			$ 97,067,000
Loans and Leases Receivable, Impaired, Loans Past Due Period, Minimum	90 days
Net credit losses (recoveries)	(66,000)		2,825,000
90-day delinquencies [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	24,661,000			28,296,000
90-day delinquencies [Member] | Eligible Loans [Member]
Financing Receivable, Impaired [Line Items]
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	1,000,000	[1]		1,200,000	[1]
On-balance sheet [Member]
Financing Receivable, Impaired [Line Items]
Net credit losses (recoveries)	(66,000)		2,825,000
On-balance sheet [Member] | Eligible Loans [Member]
Financing Receivable, Impaired [Line Items]
Net credit losses (recoveries)	(66,000)		2,825,000
On-balance sheet [Member] | 90-day delinquencies [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	21,409,000			27,580,000
On-balance sheet [Member] | 90-day delinquencies [Member] | Eligible Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	21,409,000	[1]		27,580,000	[1]
Off-balance sheet [Member]
Financing Receivable, Impaired [Line Items]
Net credit losses (recoveries)	0		0
Off-balance sheet [Member] | Long-Term Standby Purchase Commitments [Member]
Financing Receivable, Impaired [Line Items]
Net credit losses (recoveries)	0		0
Off-balance sheet [Member] | 90-day delinquencies [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	3,252,000			716,000
Off-balance sheet [Member] | 90-day delinquencies [Member] | Long-Term Standby Purchase Commitments [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 3,252,000			$ 716,000

[1] Includes loans and loans underlying off-balance sheet Farm & Ranch Guaranteed Securities and LTSPCs that are 90 days or more past due, in foreclosure, restructured after delinquency, or in bankruptcy, excluding loans performing under either their original loan terms or a court-approved bankruptcy plan.