Employee Retirement Plans - Net Pension Cost and Funded Status (Details) - Pentegra Defined Benefit Plan [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net pension cost		$ 6,559		$ 6,472		$ 6,727
Pentegra Defined Benefit Plan funded status as of July 1	[2]	108.60%	[1]	111.00%	[3]	111.80%
Our funded status as of July 1	[2]	113.90%		123.10%		122.90%

[1] The funded status as of July 1, 2019, is preliminary and may increase because the plan's participants were permitted to make contributions for the plan year ended June 30, 2019, through March 15, 2020. Contributions made on or before March 15, 2020, and designated for the plan year ended June 30, 2019, will be included in the final valuation as of July 1, 2019. The final funded status as of July 1, 2019, will not be available until the Form 5500 for the plan year July 1, 2019, through June 30, 2020 is filed. This Form 5500 is due to be filed no later than April 2021.

[2] The funded status is calculated in accordance with a provision contained in the Highway and Transportation Funding Act of 2014 (HATFA), which was signed into law on August 8, 2014, and which modifies the interest rates that had been set by the Moving Ahead for Progress in the 21st Century Act (MAP-21). MAP-21, signed into law in 2012, changed the calculation of the discount rate used in measuring the pension plan liability. MAP-21 allows plan sponsors to measure the pension plan liability using a 25-year average of interest rates plus or minus a corridor. Prior to MAP-21, the discount rate used in measuring the pension plan liability was based on the 24-month average of interest rates. HATFA amended MAP-21 by extending the time period and reducing the rate at which the 25-year corridors widen. Over time, the pension funding stabilization effect of MAP-21 will decline because the 24-month smoothed segment rates and the amended 25-year corridors are likely to converge.

[3] The funded status as of July 1, 2018, is preliminary and may increase because the plan's participants were permitted to make contributions for the plan year ended June 30, 2018, through March 15, 2019. Contributions made on or before March 15, 2019, and designated for the plan year ended June 30, 2018, will be included in the final valuation as of July 1, 2018. The final funded status as of July 1, 2018, will not be available until the Form 5500 for the plan year July 1, 2018, through June 30, 2019 is filed. This Form 5500 is due to be filed no later than April 2020.