Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets:
Settlement assets	$ 3,134.6	$ 3,296.7
Derivatives	401.2	204.5
Liabilities:
Derivatives	323.5	159.5
Recurring
Assets:
Derivatives	401.2	204.5
Total assets	2,079.3	1,927.5
Liabilities:
Derivatives	323.5	159.5
Total liabilities	323.5	159.5
Recurring \| Money market funds
Assets:
Settlement assets	24.3	24.6
Recurring \| State and municipal debt securities
Assets:
Settlement assets	1,347.6	1,257.8
Recurring \| State and municipal variable rate demand notes
Assets:
Settlement assets	207.5	276.1
Recurring \| United States government agency mortgage-backed securities
Assets:
Settlement assets	64.5	67.2
Recurring \| Corporate debt securities
Assets:
Settlement assets	34.2	52.4
Recurring \| Other United States government agency debt securities
Assets:
Settlement assets		34.9
Recurring \| United States Treasury securities
Assets:
Settlement assets		10.0
Recurring \| Level 1
Assets:
Total assets	24.3	34.6
Recurring \| Level 1 \| Money market funds
Assets:
Settlement assets	24.3	24.6
Recurring \| Level 1 \| United States Treasury securities
Assets:
Settlement assets		10.0
Recurring \| Level 2
Assets:
Derivatives	401.2	204.5
Total assets	2,055.0	1,892.9
Liabilities:
Derivatives	323.5	159.5
Total liabilities	323.5	159.5
Recurring \| Level 2 \| State and municipal debt securities
Assets:
Settlement assets	1,347.6	1,257.8
Recurring \| Level 2 \| State and municipal variable rate demand notes
Assets:
Settlement assets	207.5	276.1
Recurring \| Level 2 \| United States government agency mortgage-backed securities
Assets:
Settlement assets	64.5	67.2
Recurring \| Level 2 \| Corporate debt securities
Assets:
Settlement assets	$ 34.2	52.4
Recurring \| Level 2 \| Other United States government agency debt securities
Assets:
Settlement assets		$ 34.9

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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