Allowance for Credit Losses Schedule of Loans Outstanding and Past Due (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		$ 102,191	$ 116,067
Financing Receivable, Recorded Investment, Current		9,376,387	9,063,337
Total recorded investment		9,478,578	9,179,404
Mortgage Loans In Process Of Foreclosure	[1]	$ 17,970	$ 21,253
Loans and Leases Receivable, Serious Delinquencies Ratio	[2]	0.32%	0.40%
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	[3]	$ 27,694	$ 33,462
Financing Receivable, Recorded Investment, Nonaccrual Status		3,204	3,908
Financing Receivable, Modifications, Recorded Investment		9,648	9,889
Conventional Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		64,409	70,532
Financing Receivable, Recorded Investment, Current		9,054,781	8,712,038
Total recorded investment		9,119,190	8,782,570
Mortgage Loans In Process Of Foreclosure	[1]	$ 12,342	$ 15,412
Loans and Leases Receivable, Serious Delinquencies Ratio	[2]	0.21%	0.26%
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	[3]	$ 16,838	$ 19,408
Financing Receivable, Recorded Investment, Nonaccrual Status		3,204	3,908
Federal Housing Administration Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		37,782	45,535
Financing Receivable, Recorded Investment, Current		321,606	351,299
Total recorded investment		359,388	396,834
Mortgage Loans In Process Of Foreclosure	[1]	$ 5,628	$ 5,841
Loans and Leases Receivable, Serious Delinquencies Ratio	[2]	3.06%	3.59%
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	[3]	$ 10,856	$ 14,054
Financing Receivable, Recorded Investment, Nonaccrual Status		0	0
Past due 30-59 days delinquent
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		60,285	62,615
Past due 30-59 days delinquent | Conventional Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		39,029	39,409
Past due 30-59 days delinquent | Federal Housing Administration Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		21,256	23,206
Past due 60-89 days delinquent
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		12,558	17,625
Past due 60-89 days delinquent | Conventional Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		6,888	9,350
Past due 60-89 days delinquent | Federal Housing Administration Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		5,670	8,275
Past due 90 days or more delinquent
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		29,348	35,827
Past due 90 days or more delinquent | Conventional Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		18,492	21,773
Past due 90 days or more delinquent | Federal Housing Administration Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Recorded Investment, Past Due		$ 10,856	$ 14,054

[1] Includes loans where the decision of foreclosure or a similar alternative such as pursuit of deed-in-lieu has been reported. Loans in process of foreclosure are included in past due or current loans dependent on their delinquency status.

[2] Loans that are 90 days or more past due or in the process of foreclosure (including past due or current loans in the process of foreclosure) expressed as a percentage of the total loan portfolio class recorded investment amount.

[3] Each conventional loan past due 90 days or more still accruing interest is on a schedule/scheduled monthly settlement basis and contains one or more credit enhancements. Loans that are well secured and in the process of collection as a result of remaining credit enhancements and schedule/scheduled settlement are not placed on non-accrual status.