Trading Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	$ 1,292,432	$ 462,676	$ 677,156
Trading Securities, Net Unrealized Gain (Loss)	(2,663)	(448)	1,708
U.S. government agency debentures
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	37,115	21,196	18,365
Trading Securities, Net Unrealized Gain (Loss)	68	8	(74)
U.S. government agency residential mortgage-backed securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	1,078,085	392,673	578,977
Trading Securities, Net Unrealized Gain (Loss)	(2,692)	(517)	1,575
Municipal and other tax-exempt securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	72,013	13,559	45,114
Trading Securities, Net Unrealized Gain (Loss)	0	83	171
Asset-backed securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	94,734	23,885	0
Trading Securities, Net Unrealized Gain (Loss)	19	(26)	0
Other trading securities
Schedule of Trading Securities [Line Items]
Trading Securities, Fair Value	10,485	11,363	34,700
Trading Securities, Net Unrealized Gain (Loss)	$ (58)	$ 4	$ 36