Trading Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	$ 1,909,615	$ 462,676	$ 441,414
Trading Securities, Net Unrealized Gain (Loss)	2,446	(448)	(986)
Trading Securities Pledged as Collateral	889,000	0	0
U.S. government agency debentures
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	28,750	21,196	20,954
Trading Securities, Net Unrealized Gain (Loss)	10	8	(9)
U.S. government agency residential mortgage-backed securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	1,605,001	392,673	365,171
Trading Securities, Net Unrealized Gain (Loss)	1,923	(517)	(1,032)
Municipal and other tax-exempt securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	70,606	13,559	45,444
Trading Securities, Net Unrealized Gain (Loss)	231	83	230
Asset-backed securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	193,271	23,885	0
Trading Securities, Net Unrealized Gain (Loss)	250	(26)	0
Other trading securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	11,987	11,363	9,845
Trading Securities, Net Unrealized Gain (Loss)	$ 32	$ 4	$ (175)