Financial instruments - Derivative and non-derivative financial instruments (Details) - EUR (€) € in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Reconciliation of changes in fair value measurement
Assets at beginning of period	€ 32,934,735
Assets at end of period	34,072,427	€ 32,934,735
Liabilities at beginning of period	19,707,498
Liabilities at end of period	20,802,491	19,707,498
Equity investments \| Fair Value \| Level 3
Reconciliation of changes in fair value measurement
Assets at beginning of period	183,054
Transfer from Level 2		186,427
Increase		2,233
Gains (losses) recognised in profit or loss	(20,843)	128
Foreign currency translation and other changes	4,511	(5,734)
Assets at end of period	166,722	183,054
Variable payments outstanding for acquisition \| Fair Value \| Level 3
Reconciliation of changes in fair value measurement
Liabilities at beginning of period	89,677	172,278
Increase	11,886	4,828
Decrease	(26,229)	(43,941)
(Gain) loss recognized in profit or loss	12	(41,537)
Foreign currency translation and other changes	(1,152)	(1,951)
Liabilities at end of period	74,194	89,677
Noncontrolling interests subject to put provisions \| Fair Value \| Level 3
Reconciliation of changes in fair value measurement
Liabilities at beginning of period	934,425	818,871
Increase	5,234	109,109
Decrease	(8,720)	(20,269)
(Gain) loss recognized in equity	12,963	14,523
Foreign currency translation and other changes	9,817	12,191
Liabilities at end of period	€ 953,719	€ 934,425
Noncontrolling interests subject to put provisions \| Fair Value \| Level 3 \| Assumed earnings
Reconciliation of changes in fair value measurement
Increase in input	10.00%
Increase in fair value due to increase in input	€ 68,490
Noncontrolling interests subject to put provisions \| Fair Value \| Level 3 \| Assumed earnings \| Maximum
Reconciliation of changes in fair value measurement
Increase in fair value due to increase in input, as percentage of total liabilities	1.00%
Increase in fair value due to increase in input, as percentage of equity	1.00%

The amount (gain) loss recognized in equity.

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Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, as percentage of equity

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Increase (decrease) in fair value measurement due to reasonably possible increase in unobservable input, as percentage of total liabilities

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Increase (decrease) through net exchange differences, financial assets

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The increase (decrease) in financial liabilities resulting from the net exchange differences arising when the financial statements are translated from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity.

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Expiry date 2020-01-01: The amount of resources: (a) controlled by the entity as a result of past events; and (b) from which future economic benefits are expected to flow to the entity. Effective 2020-01-01: The amount of a present economic resource controlled by the entity as a result of past events. Economic resource is a right that has the potential to produce economic benefits.

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The gains (losses) excluding exchange differences, recognised in profit or loss, on the fair value measurement of assets. [Refer: At fair value [member]]

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The gains (losses) excluding exchange differences, recognised in profit or loss, on the fair value measurement of liabilities. [Refer: At fair value [member]]

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The amount of increase (decrease) in the fair value measurement of liabilities due to a reasonably possible increase in an unobservable input.

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The increase in fair value measurement of liabilities resulting from issues of those liabilities. [Refer: At fair value [member]]

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Expiry date 2020-01-01: The amount of present obligations of the entity arising from past events, the settlement of which is expected to result in an outflow from the entity of resources embodying economic benefits. Effective 2020-01-01: The amount of a present obligation of the entity to transfer an economic resource as a result of past events. Economic resource is a right that has the potential to produce economic benefits.

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The percentage of a reasonably possible increase in an unobservable input used in fair value measurement of liabilities.

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The increase in the fair value measurement of assets resulting from purchases of those assets. [Refer: At fair value [member]]

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The decrease in the fair value measurement of liabilities resulting from settlements. [Refer: At fair value [member]]

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The amount of transfers of assets into Level 3 of the fair value hierarchy. [Refer: Level 3 of fair value hierarchy [member]]

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