Securities - Schedule of Estimated Fair Values of Securities with Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Estimated Fair Value
Less Than Twelve Months	$ 143,041	$ 169,584
Twelve Months or Longer	19,862	20,212
Total	162,903	189,796
Gross Unrealized Losses
Less Than Twelve Months	(3,746)	(3,012)
Twelve Months or Longer	(767)	(833)
Total	(4,513)	(3,845)
Estimated Fair Value
Less Than Twelve Months	1,806,405	1,825,723
Twelve Months or Longer	252,195	231,385
Total	2,058,600	2,057,108
Gross Unrealized Losses
Less Than Twelve Months	(44,848)	(47,248)
Twelve Months or Longer	(8,847)	(8,316)
Total	(53,695)	(55,564)
GSE issuance REMICs and CMOs
Estimated Fair Value
Less Than Twelve Months	113,064	140,638
Twelve Months or Longer	19,720	20,026
Total	132,784	160,664
Gross Unrealized Losses
Less Than Twelve Months	(2,634)	(1,886)
Twelve Months or Longer	(752)	(820)
Total	(3,386)	(2,706)
Estimated Fair Value
Less Than Twelve Months	569,367	515,537
Twelve Months or Longer	125,051	131,629
Total	694,418	647,166
Gross Unrealized Losses
Less Than Twelve Months	(7,110)	(7,457)
Twelve Months or Longer	(4,201)	(4,500)
Total	(11,311)	(11,957)
Non-GSE issuance REMICs and CMOs
Estimated Fair Value
Less Than Twelve Months	1,022	0
Twelve Months or Longer	81	92
Total	1,103	92
Gross Unrealized Losses
Less Than Twelve Months	(4)	0
Twelve Months or Longer	(1)	(1)
Total	(5)	(1)
Estimated Fair Value
Less Than Twelve Months	0	0
Twelve Months or Longer	184	186
Total	184	186
Gross Unrealized Losses
Less Than Twelve Months	0	0
Twelve Months or Longer	(7)	(7)
Total	(7)	(7)
GSE pass-through certificates
Estimated Fair Value
Less Than Twelve Months	62	71
Twelve Months or Longer	59	90
Total	121	161
Gross Unrealized Losses
Less Than Twelve Months	(1)	(1)
Twelve Months or Longer	(1)	(1)
Total	(2)	(2)
Estimated Fair Value
Less Than Twelve Months	81,074	104,538
Twelve Months or Longer	60,450	61,872
Total	141,524	166,410
Gross Unrealized Losses
Less Than Twelve Months	(1,235)	(1,775)
Twelve Months or Longer	(1,149)	(1,507)
Total	(2,384)	(3,282)
Obligations of GSEs
Estimated Fair Value
Less Than Twelve Months	28,893	28,875
Twelve Months or Longer	0	0
Total	28,893	28,875
Gross Unrealized Losses
Less Than Twelve Months	(1,107)	(1,125)
Twelve Months or Longer	0	0
Total	(1,107)	(1,125)
Estimated Fair Value
Less Than Twelve Months	311,690	296,427
Twelve Months or Longer	0	0
Total	311,690	296,427
Gross Unrealized Losses
Less Than Twelve Months	(16,249)	(16,510)
Twelve Months or Longer	0	0
Total	(16,249)	(16,510)
GSE issuance REMICs
Estimated Fair Value
Less Than Twelve Months	834,947	871,436
Twelve Months or Longer	0	0
Total	834,947	871,436
Gross Unrealized Losses
Less Than Twelve Months	(19,581)	(19,290)
Twelve Months or Longer	0	0
Total	(19,581)	(19,290)
Fannie Mae stock
Estimated Fair Value
Less Than Twelve Months	0	0
Twelve Months or Longer	2	4
Total	2	4
Gross Unrealized Losses
Less Than Twelve Months	0	0
Twelve Months or Longer	(13)	(11)
Total	(13)	(11)
Corporate debt securities
Estimated Fair Value
Less Than Twelve Months	9,327	37,785
Twelve Months or Longer	66,510	37,698
Total	75,837	75,483
Gross Unrealized Losses
Less Than Twelve Months	(673)	(2,216)
Twelve Months or Longer	(3,490)	(2,302)
Total	$ (4,163)	$ (4,518)