Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 position	Dec. 31, 2012 position
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 5,518,866	$ 5,749,626
Available-for-sale Securities, Gross Unrealized Gains	4,992	23,448
Available-for-sale Securities, Gross Unrealized Losses	7,779	921
Available-for-sale Securities, Debt Securities	5,516,079	5,772,153
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Positions	145	42
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,412,755	600,614
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	7,670	906
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Positions	3	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	14,850	2,373
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	109	15
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	148	43
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,427,605	602,987
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	7,779	921
Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost Basis	5,016	0
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	3,871,921	3,340,880
Available-for-sale Securities, Debt Maturities, Rolling Year Six Through Ten, Amortized Cost Basis	1,641,929	2,401,976
Available-for-sale Securities, Debt Maturities, Rolling after Year Ten, Amortized Cost Basis	0	6,770
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	5,518,866	5,749,626
Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Fair Value	5,019	0
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	3,874,920	3,349,030
Available-for-sale Securities, Debt Maturities, Rolling Year Six Through Ten, Fair Value	1,636,140	2,416,299
Available-for-sale Securities, Debt Maturities, Rolling after Year Ten, Fair Value	0	6,824
Fixed-rate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	5,443,866	5,674,626
Variable-rate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	75,000	75,000
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	54,564	56,119
Available-for-sale Securities, Gross Unrealized Gains	62	301
Available-for-sale Securities, Gross Unrealized Losses	108	3
Available-for-sale Securities, Debt Securities	54,518	56,417
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Positions	3	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	35,198	11,340
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	108	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Positions	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	3	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	35,198	11,340
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	108	3
US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	5,023,165	5,236,358
Available-for-sale Securities, Gross Unrealized Gains	4,570	22,280
Available-for-sale Securities, Gross Unrealized Losses	6,440	644
Available-for-sale Securities, Debt Securities	5,021,295	5,257,994
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Positions	119	23
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,134,921	448,123
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	6,440	629
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Positions	0	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	2,373
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	15
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	119	24
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,134,921	450,496
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	6,440	644
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	441,137	457,149
Available-for-sale Securities, Gross Unrealized Gains	360	867
Available-for-sale Securities, Gross Unrealized Losses	1,231	274
Available-for-sale Securities, Debt Securities	440,266	457,742
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Number of Positions	23	18
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	242,636	141,151
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1,122	274
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Number of Positions	3	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	14,850	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	109	0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	26	18
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	257,486	141,151
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 1,231	$ 274