Components of Short-Term and Long-Term Debt (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Marquette Capital Trust I subordinated debentures 2.69% due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	2.69%	2.69%
Debt instrument, maturity year	2036	2036
Marquette Capital Trust II subordinated debentures 2.69% due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	2.69%	2.69%
Debt instrument, maturity year	2036	2036
Marquette Capital Trust III subordinated debentures 3.17% due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	3.17%	3.17%
Debt instrument, maturity year	2036	2036
Marquette Capital Trust IV subordinated debentures 2.92% due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	2.92%	2.92%
Debt instrument, maturity year	2036	2036
Kansas Equity Fund IV, L.P. 0% due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2017	2017
Kansas Equity Fund V, L.P. 0% due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2017	2017
Kansas Equity Fund VI, L.P. 0% due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2017	2017
Kansas Equity Fund IX, L.P. 0% due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2023	2023
Kansas Equity Fund X, L.P. 0% due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2021	2021
Kansas City Equity Fund 2009, L.L.C. 0% due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2017	2017
St. Louis Equity Fund 2007 L.L.C. 0% due 2016 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2019	2019
St. Louis Equity Fund 2009 L.L.C. 0% due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2017	2017
St. Louis Equity Fund 2012 L.L.C. 0% due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2020	2020
St. Louis Equity Fund 2013 L.L.C. 0% due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2021	2021
St. Louis Equity Fund 2014 L.L.C. 0% due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2022	2022
St. Louis Equity Fund 2015 L.L.C. 0% due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2023	2023
MHEG Community Fund 41, L.P. 0% due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2024	2024
MHEG Community Fund 43, L.P. 0% due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2026	2026
MHEG Community Fund 45, L.P. 0% due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2027	2027
MHEG Community Fund 47, L.P. 0% due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2028	2028
MHEG Community Fund 49, L.P. 0% due 2034 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity year	2034	2034