Securities - Schedule of Estimated Fair Values of Securities with Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Less Than Twelve Months
Estimated Fair Value	$ 169,584	$ 214,138
Gross Unrealized Losses	(3,012)	(3,327)
Twelve Months or Longer
Estimated Fair Value	20,212	47,506
Gross Unrealized Losses	(833)	(1,391)
Total
Estimated Fair Value	189,796	261,644
Gross Unrealized Losses	(3,845)	(4,718)
Less Than Twelve Months
Estimated Fair Value	1,825,723	895,094
Gross Unrealized Losses	(47,248)	(8,865)
Twelve Months or Longer
Estimated Fair Value	231,385	396,576
Gross Unrealized Losses	(8,316)	(12,988)
Total
Estimated Fair Value	2,057,108	1,291,670
Gross Unrealized Losses	(55,564)	(21,853)
GSE issuance REMICs and CMOs
Less Than Twelve Months
Estimated Fair Value	140,638	189,364
Gross Unrealized Losses	(1,886)	(2,934)
Twelve Months or Longer
Estimated Fair Value	20,026	0
Gross Unrealized Losses	(820)	0
Total
Estimated Fair Value	160,664	189,364
Gross Unrealized Losses	(2,706)	(2,934)
Less Than Twelve Months
Estimated Fair Value	515,537	395,659
Gross Unrealized Losses	(7,457)	(3,972)
Twelve Months or Longer
Estimated Fair Value	131,629	289,645
Gross Unrealized Losses	(4,500)	(10,156)
Total
Estimated Fair Value	647,166	685,304
Gross Unrealized Losses	(11,957)	(14,128)
Non-GSE issuance REMICs and CMOs
Less Than Twelve Months
Estimated Fair Value	0	75
Gross Unrealized Losses	0	(2)
Twelve Months or Longer
Estimated Fair Value	92	64
Gross Unrealized Losses	(1)	(5)
Total
Estimated Fair Value	92	139
Gross Unrealized Losses	(1)	(7)
Less Than Twelve Months
Estimated Fair Value	0	0
Gross Unrealized Losses	0	0
Twelve Months or Longer
Estimated Fair Value	186	193
Gross Unrealized Losses	(7)	(5)
Total
Estimated Fair Value	186	193
Gross Unrealized Losses	(7)	(5)
GSE pass-through certificates
Less Than Twelve Months
Estimated Fair Value	71	97
Gross Unrealized Losses	(1)	(1)
Twelve Months or Longer
Estimated Fair Value	90	103
Gross Unrealized Losses	(1)	(1)
Total
Estimated Fair Value	161	200
Gross Unrealized Losses	(2)	(2)
Less Than Twelve Months
Estimated Fair Value	104,538	56,503
Gross Unrealized Losses	(1,775)	(586)
Twelve Months or Longer
Estimated Fair Value	61,872	106,738
Gross Unrealized Losses	(1,507)	(2,827)
Total
Estimated Fair Value	166,410	163,241
Gross Unrealized Losses	(3,282)	(3,413)
GSE issuance REMICs
Less Than Twelve Months
Estimated Fair Value	871,436	276,601
Gross Unrealized Losses	(19,290)	(2,334)
Twelve Months or Longer
Estimated Fair Value	0	0
Gross Unrealized Losses	0	0
Total
Estimated Fair Value	871,436	276,601
Gross Unrealized Losses	(19,290)	(2,334)
Obligations of GSEs
Less Than Twelve Months
Estimated Fair Value	28,875	24,602
Gross Unrealized Losses	(1,125)	(390)
Twelve Months or Longer
Estimated Fair Value	0	47,337
Gross Unrealized Losses	0	(1,372)
Total
Estimated Fair Value	28,875	71,939
Gross Unrealized Losses	(1,125)	(1,762)
Less Than Twelve Months
Estimated Fair Value	296,427	107,824
Gross Unrealized Losses	(16,510)	(480)
Twelve Months or Longer
Estimated Fair Value	0	0
Gross Unrealized Losses	0	0
Total
Estimated Fair Value	296,427	107,824
Gross Unrealized Losses	(16,510)	(480)
Fannie Mae stock
Less Than Twelve Months
Estimated Fair Value	0	0
Gross Unrealized Losses	0	0
Twelve Months or Longer
Estimated Fair Value	4	2
Gross Unrealized Losses	(11)	(13)
Total
Estimated Fair Value	4	2
Gross Unrealized Losses	(11)	(13)
Corporate debt securities
Less Than Twelve Months
Estimated Fair Value	37,785	58,507
Gross Unrealized Losses	(2,216)	(1,493)
Twelve Months or Longer
Estimated Fair Value	37,698	0
Gross Unrealized Losses	(2,302)	0
Total
Estimated Fair Value	75,483	58,507
Gross Unrealized Losses	$ (4,518)	$ (1,493)