Securities - Estimated Fair Values of Securities with Gross Unrealized Losses in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Less Than Twelve Months
Estimated Fair Value	$ 214,138	$ 45,226
Gross Unrealized Losses	(3,327)	(555)
Twelve Months or Longer
Estimated Fair Value	47,506	147,718
Gross Unrealized Losses	(1,391)	(2,999)
Total
Estimated Fair Value	261,644	192,944
Gross Unrealized Losses	(4,718)	(3,554)
Less Than Twelve Months
Estimated Fair Value	895,094	222,216
Gross Unrealized Losses	(8,865)	(892)
Twelve Months or Longer
Estimated Fair Value	396,576	744,508
Gross Unrealized Losses	(12,988)	(19,146)
Total
Estimated Fair Value	1,291,670	966,724
Gross Unrealized Losses	(21,853)	(20,038)
GSE issuance REMICs and CMOs
Less Than Twelve Months
Estimated Fair Value	189,364	20,587
Gross Unrealized Losses	(2,934)	(159)
Twelve Months or Longer
Estimated Fair Value	0	75,444
Gross Unrealized Losses	0	(1,397)
Total
Estimated Fair Value	189,364	96,031
Gross Unrealized Losses	(2,934)	(1,556)
Less Than Twelve Months
Estimated Fair Value	395,659	121,861
Gross Unrealized Losses	(3,972)	(302)
Twelve Months or Longer
Estimated Fair Value	289,645	500,348
Gross Unrealized Losses	(10,156)	(13,739)
Total
Estimated Fair Value	685,304	622,209
Gross Unrealized Losses	(14,128)	(14,041)
Non-GSE issuance REMICs and CMOs
Less Than Twelve Months
Estimated Fair Value	75	0
Gross Unrealized Losses	(2)	0
Twelve Months or Longer
Estimated Fair Value	64	96
Gross Unrealized Losses	(5)	(1)
Total
Estimated Fair Value	139	96
Gross Unrealized Losses	(7)	(1)
Less Than Twelve Months
Estimated Fair Value	0	0
Gross Unrealized Losses	0	0
Twelve Months or Longer
Estimated Fair Value	193	294
Gross Unrealized Losses	(5)	(7)
Total
Estimated Fair Value	193	294
Gross Unrealized Losses	(5)	(7)
GSE pass-through certificates
Less Than Twelve Months
Estimated Fair Value	97	53
Gross Unrealized Losses	(1)	(1)
Twelve Months or Longer
Estimated Fair Value	103	64
Gross Unrealized Losses	(1)	(1)
Total
Estimated Fair Value	200	117
Gross Unrealized Losses	(2)	(2)
Less Than Twelve Months
Estimated Fair Value	56,503	0
Gross Unrealized Losses	(586)	0
Twelve Months or Longer
Estimated Fair Value	106,738	164,453
Gross Unrealized Losses	(2,827)	(3,877)
Total
Estimated Fair Value	163,241	164,453
Gross Unrealized Losses	(3,413)	(3,877)
GSE issuance REMICs
Less Than Twelve Months
Estimated Fair Value	276,601	100,355
Gross Unrealized Losses	(2,334)	(590)
Twelve Months or Longer
Estimated Fair Value	0	0
Gross Unrealized Losses	0	0
Total
Estimated Fair Value	276,601	100,355
Gross Unrealized Losses	(2,334)	(590)
Obligations of GSEs
Less Than Twelve Months
Estimated Fair Value	24,602	24,586
Gross Unrealized Losses	(390)	(395)
Twelve Months or Longer
Estimated Fair Value	47,337	72,112
Gross Unrealized Losses	(1,372)	(1,587)
Total
Estimated Fair Value	71,939	96,698
Gross Unrealized Losses	(1,762)	(1,982)
Less Than Twelve Months
Estimated Fair Value	107,824	0
Gross Unrealized Losses	(480)	0
Twelve Months or Longer
Estimated Fair Value	0	79,413
Gross Unrealized Losses	0	(1,523)
Total
Estimated Fair Value	107,824	79,413
Gross Unrealized Losses	(480)	(1,523)
Fannie Mae stock
Less Than Twelve Months
Estimated Fair Value	0	0
Gross Unrealized Losses	0	0
Twelve Months or Longer
Estimated Fair Value	2	2
Gross Unrealized Losses	(13)	(13)
Total
Estimated Fair Value	2	2
Gross Unrealized Losses	(13)	$ (13)
Corporate debt securities
Less Than Twelve Months
Estimated Fair Value	58,507
Gross Unrealized Losses	(1,493)
Twelve Months or Longer
Estimated Fair Value	0
Gross Unrealized Losses	0
Total
Estimated Fair Value	58,507
Gross Unrealized Losses	$ (1,493)